1 ActionScript 3.0 Language Specification This document defines the ActionScript 3.0 language, which is designed to be forward- compatible with the next edition of ECMAScript (ECMA-262). This document is complete in the syntax and high-level semantics of the language, but lacks details regarding features that have not changed from the ECMA-262 edition 3 specification, compatibility with ActionScript 2.0, and low-level semantics of some new features Source http://livedocs.adobe.com/specs/actionscript/3/wwhelp/wwhimpl/js/html/wwhelp.htm
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ActionScript 3.0 Language Specification
This document defines the ActionScript 3.0 language, which is designed to be forward- compatible with the next edition of ECMAScript (ECMA-262).
This document is complete in the syntax and high-level semantics of the language, but
lacks details regarding features that have not changed from the ECMA-262 edition 3 specification, compatibility with ActionScript 2.0, and low-level semantics of some new
An ActionScript program consists of zero or more package definitions followed by zero or more directives, which includes non-package definitions and statements. Statements
inside and outside of package definitions are evaluated in order, independent of their
nesting inside a package.
1.1 Hello world
The following sections show various ways to implement simple programs such as the
familiar 'hello, world' program in ActionScript 3.0:
trace("hello, world");
This is a single expression statement that calls a function named trace() with the argument that is a literal string "hello, world". An expression statement does
nothing but execute an expression.
1.2 Expressions
Here are some examples of expressions:
x = 1 + 2
x = y()
x = y..z
x = o.ns::id
Expressions evaluate to values:
1+2 evaluates to 3.
y() evaluates to the result of calling the function y with no arguments.
y..z evaluates to the set of all properties identified by z in the value of y and the
descendants of y. The descendants accessor operator (..) is part of the ActionScript
3.0 implementation of ECMAScript for XML (E4X).
o.ns::id evaluates to the value of the property ns::id of the value of o, where o represents an object, ns represents a namespace, and id represents an
identifier.
1.3 Statements
Statements are executed in the order that they appear in a block. Some statements
change control flow by abrupt completion, such as break and continue, or by iteration,
such as while and do. An example of a statement follows:
for (var i:int = 0; i < 5; i++) {
trace(i);
}
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1.4 Variables
Variables define properties whose values can change at runtime. They can be defined
with either the var keyword or the constkeyword. A variable that is defined with
the var keyword may be assigned by any code that can access it. A variable that is defined with the const keyword may only be set by its initializer, or its class's instance
constructor if it is an instance variable. An example of variables follows:
var x = 10
const PI = 3.1415
1.5 Functions
Functions define properties whose values can be called. An example of a function follows:
function hello() {
trace("hello, world")
}
hello()
Depending on where a function is defined, it results in a property whose value is a
function closure or a method. A function closure is a first class object that can be treated as a collection of properties or a callable object. A method is tightly bound to
the object that it is associated with. The this reference of a function is bound to the
base object of the call expression, or the global object if none is specified.
function hello() {
trace("hello, world")
trace(this) // this refers to global object
}
hello()
A method is a function that is tightly bound to an object. A method can be extracted
from its instance, but, unlike function closures, the value of this always refers to the
instance it is extracted from.
1.6 Classes
A class is an object that can be used as a constructor of instances that share the same
type and properties. An example of a class follows:
class Greeter {
var saying = "hello, world"
function hello() {
trace(saying)
}
}
var greeter : Greeter = new Greeter
greeter.hello()
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Class definitions are used to define the fixed properties of a class object. Property
definitions that are marked with the staticattribute become properties of the class object, and those that are not become properties of instances of the class.
Class and instance properties are either methods or slots. A method is defined by a
function definition inside a class definition. A method has a definition (called a method trait) that is shared among all instances of the same type. Unlike an ordinary function
object, a method is tightly bound to the object it is associated with. Whenever and
however it gets invoked, the meaning of the expressionthis is always the same. In fact, methods can be extracted from their instance and treated as first class objects
(called bound methods), much like function objects can be. There is one important
difference between a function closure and a bound method. With a bound method,
the this reference is bound into the object so that whenever it is invoked the original this reference is used. With a function closure, this is generic and refers to
any object the function happens to be associated with when it is invoked.
Slots are defined by variable definitions inside a class definition. An instance variable has a definition (called a slot trait) that is shared among all instances of the same type,
but a unique location in each object.
1.7 Interfaces
An interface defines a contract between an instance and code that uses that instance.
When a class implements an interface, it guarantees that it will provide the methods
declared in that interface. An implementing method must be declared public, in which case it will implement all unimplemented interface methods with the same identifier. An
example of an interface follows:
interface Greetings {
function hello()
function goodmorning()
}
class Greeter implements Greetings {
public function hello() {
trace("hello, world")
}
public function goodmorning() {
trace("goodmorning, world")
}
}
var greeter : Greetings = new Greeter()
greeter.hello()
1.8 Packages
Packages in ActionScript are very similar to packages in Java and namespaces in C# and C++. Packages are useful for organizing frameworks (or toolkits, or APIs) into sets
of related definitions: classes, namespaces, interfaces, functions, and variables.
Client code can import all or parts of a package to access the functionality it provides without cluttering its global namespace with unneeded names. In the following
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example, the import directive makes the class Greeter visible to the global code that
contains the import directive.
package actors {
public class Greeter {
public function hello() {
trace("hello, world")
}
}
}
import actors.Greeter
var greeter : Greeter = new Greeter
greeter.hello()
1.9 Namespaces
Namespaces are useful for controlling the visibility of a set of properties independent of
the major structure of the program. Packages, classes and interfaces, along with their
implicitly defined access control namespaces, allow authors to control the visibility of names in parallel with the organization of those packages, classes and interfaces. But it
is sometimes necessary to control the names independent of the lexical structure of a
program. Examples of this include the following:
Making the public interface of a set of classes look different to different client modules
Evolving a class over time without changing the behavior of existing programs
Providing privileged access to a limited set of clients
Use packages to give or gain access to a set of features. Use namespaces to give or
gain access to a particular facet, version, or privilege independent of the structure of a
program. An example that uses namespaces follows:
// ActionScript file: actors/English.as
package actors{
public namespace English =
"http://www.adobe.com/2007/Examples/English";
}
// ActionScript file: actors/French.as
package actors {
public namespace French = "http://www.adobe.com/2007/Examples/French";
}
// ActionScript file: actors/BilingualGreeter.as
package actors {
public class BilingualGreeter {
English function hello():void {
trace("hello, world");
}
French function hello():void {
trace("bonjour, le monde");
}
}
}
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// ActionScript file: Test.as
package {
import flash.display.Sprite;
public class Test extends Sprite
{
public function Test()
{
}
}
}
import actors.*;
var greeter : BilingualGreeter = new BilingualGreeter();
use namespace English; // Make all identifiers in the English namespace
// visible
greeter.hello(); // Invoke the English version
greeter.French::hello(); // Invoke the French version
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2 Design perspective
It is sometimes difficult to understand design decisions without understanding the perspective of the designers. Here are the major viewpoints that have grounded the
design changes introduced in ActionScript 3.0 and ECMA-262 edition 4.
2.1 Compatibility with existing programs
ECMAScript was originally designed for and used by consumers of host object models. Because ECMAScript is one of the most widely used programming languages, it is
important that existing ECMAScript-compliant programs continue to work in systems
that are updated to support the new definition of the language.
Therefore, programs written for ECMA-262 edition 3, compact profile, or ECMAScript for
XML (ECMA-357 edition 2, also known as E4X) must continue to behave the same way
in both ActionScript 3.0 and ECMA-262 edition 4.
2.2 Compatibility with existing object models
Through 10 years of use, ECMAScript has come under great pressure to become a
language for creating object models. This is a natural consequence of the need for application and tool developers to extend and override the functionality of the built-in
objects provided by host environments. A few examples of this include HTML, Flash,
Acrobat, and VoiceXML.
These embeddings contain host objects with behaviors that can only be approximated with the features of ECMA-262 edition 3, and as such are implemented in a way that is
inefficient and fragile.
Therefore, one of the mandates of edition 4 is to make it possible to create object models, such as the ECMA-262 edition 3 built-ins, HTML DOM and ActionScript API, in a
way that not only makes it natural to give these object models behavior like the
existing object models, but that also makes them robust and efficient.
2.3 Controlling the visibility of names
It is a well-known problem that naming conflicts arise when independently created
libraries are used by a single application. It is also common that the meaning of a name must be different for different uses of a single component.
Therefore, edition 4 strives to minimize the occurrence of naming conflicts when
independently created libraries are used by a single application and make it possible to
resolve those conflicts when they do occur. Furthermore, edition 4 strives to make it possible for users to select the meaning of names between versions and uses.
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2.4 Choosing between reliability and flexibility
Whereas the original purpose of ECMAScript was to provide a scripting language for
automating web pages and other hosted applications where lenient runtime behavior is
preferred and scripts are small enough that performance is often not a concern, libraries written in ECMAScript can be very large and complex, and be constrained by
aggressive performance requirements. These libraries are often created ahead of time
using IDEs and stand-alone compilers. In this case, developers are willing to give up
some flexibility to be guaranteed that certain kinds of errors will not occur at runtime, and that their code will run as efficiently as possible.
Also, it is desirable when targeting low-powered platforms to minimize the amount of
processing that must occur to execute programs on the client.
Therefore, edition 4 allows developers to trade flexibility and compatibility for reliability
and efficiency by choosing a well-defined subset of ECMAScript that can be compiled
ahead-of-time for more aggressive compile-time semantic analysis and optimization.
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3 Phases and dialects of interpretation
There are three phases of execution: parsing, verification and evaluation. Invalid programs will terminate during one of these three phases, before the program runs to
completion.
There are two dialects of the language described by this specification, one a subset of the other. These languages differ only in that one has additional verification rules. The
more permissive language is called the standard dialect, and the more restrictive
language is called the strict dialect.
3.1 Parsing
The parsing phase translates the source code of a program into an internal format
suitable for verification. The syntax rules of the language are described using grammar
productions throughout this specification.
3.2 Verifying
The verification phase ensures that the program obeys the static semantics of the
language. In the standard dialect, verification may be done anytime before a construct is first evaluated. In the strict dialect, verification must happen before any part of the
program is evaluated.
The differences in the verification rules of the standard dialect and the strict dialect
mean that some programs that would verify in the standard language will not verify in the strict language. However, all programs that verify in the strict language will verify
and run with the same behavior in the standard language.
3.2.1 Compile time constant expressions
A compile time constant expression is an expression whose value can be determined at
compile time (during verification), before any part of the program has been executed.
Compile time constant expressions consist of the following sub-expressions:
Literals such as null, Number, Boolean and String literals
References to properties whose values are compile-time constants
Operators whose results can be computed at compile time
Expressions in certain contexts are required to be compile time constant expressions:
Type annotations
Inheritance clauses references
Attributes
Pragma arguments (for example, use namespace ns2)
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Of these, inheritance clause references and attributes must not have forward
references.
3.3 Evaluating
The evaluation phase takes the parsed, verified program and evaluates it to produce
side effects in its host environment and a final value. The semantics of evaluation are
the same for both dialects of the language.
3.4 Strict verification
The goal of strict mode is reliability of new programs. The strict language is a subset of
the standard language and has three kinds of constraints:
Expressions have static types and type errors are verification errors
Common programming errors are caught by additional verification rules
Verification errors are reported ahead-of-time
3.4.1 Type errors
Here is an example of a program that is valid in the standard dialect but not valid in the strict dialect:
class A {}
class B extends A {}
var a : A = new B
var b : B = a // type error, static type of 'a' is A,
// which is incompatible with B
In the standard dialect this program has no error, since type errors are runtime errors
and the runtime value of a is an instance of B, which is clearly a member of the type B.
3.4.2 Strict errors
The strict dialect adds various semantic errors to catch common programming mistakes
that are allowed in the standard dialect for the sake of compatibility and flexibility.
Verification errors of strict mode fall into these categories:
Function call signature matching
Duplicate definition conflicts
Unbound references
Dynamic addition of properties on sealed objects
Writing to const variables
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Deleting fixed properties
Comparison expressions with incompatible types
Unfound packages
4 Definitions
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4 Definitions
This section defines terms used elsewhere in this specification.
4.1 Bound method
A bound method is a method that is extracted from the instance to which it is attached.
This typically occurs when a method is passed as an argument to a function. Such a
method is bound to the original instance in that the this reference continues to refer to that instance.
4.2 Class
Every class definition is represented by a special class object that stores information about the class. Among the constituents of the class object are two traits objects and a
prototype object. One traits object stores information about the static properties of the
class. The other traits object stores information about the instance properties of the class and serves as the primary mechanism for class inheritance. The prototype object
is a special object that can be used to share state among all instances of a class.
4.3 Class method
A class method, also called a static method, is a method that is attached to an entire class, rather than to an instance of a class. Class methods, unlike instance methods,
can only be accessed through the class, and cannot be accessed through a class
instance.
4.4 Class variable
A class variable, also called a static variable, is a variable that is attached to a class
object rather than to an instance of the class. Class variables, unlike instance variables, can only be accessed through the class, and cannot be accessed through a class
instance.
4.5 Delegate
Delegates are objects that can substitute for other objects during property name lookup. Every object has a delegate, which is either of the same type as that object or
of type Object. An instance of a class is an example of an object that has a delegate of
the same type. Class instances all share the same delegate--the defining class's prototype object. A class's prototype object is a special instance of that class that
provides a mechanism for sharing state across all instances of a class.
At runtime, when a property is not found on a class instance, the delegate, which is the class prototype object, is checked for that property. If the prototype object does not
contain the property, the process continues with the prototype object's delegate. A
prototype object is an example of an object that has a delegate of type Object. All class
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prototype objects share the same delegate--a special static property of the Object class
named Object.prototype.
4.6 Final
A class declared as final cannot be extended. A method declared as final cannot be
overridden.
4.7 Function
A function is a callable object. A function can be either a function closure or a method,
depending on how the function is defined.
4.8 Function Closure
A function closure is a function that is neither attached to another object nor defined as
part of a class. Function closures are first-class objects that can be treated as a
collection of properties or as callable objects. Contrast function closures with methods, which are functions that are attached to an object or an instance of a class.
4.9 Instance
An instance is an object that is created using a class definition.
4.10 Instance method
An instance method is a method defined without the static attribute. Instance
methods attach to a class instance instead of to the class as a whole.
4.11 Instance variable
An instance variable is a variable defined without the static attribute. Instance variables attach to a class instance instead of to the class as a whole.
4.12 Method
A method is a function that is attached to an object or an instance of a class. Contrast with function closures, which are functions not attached to an object or an instance of a
class.
4.13 Object
Every value visible in a program is an object. An object is a collection of properties.
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4.14 Property
A property associates a name with a value or method. A method can be either a get or
set accessor or an ordinary method. Fixed properties cannot be redefined or deleted.
Dynamic properties are created at runtime and can be redefined and deleted. Internally, fixed properties are expressed as traits. Dynamic properties are expressed
as a map between names and values.
4.15 Prototype
A prototype object is a special class instance that is stored internally by a class object. It is an object that becomes the implicit delegate shared by all instances of a particular
class or function. A class prototype is an instance of that class, while the prototype's
delegate is an instance of Object.
4.16 Sealed
An object is sealed if properties cannot be added to it at runtime. By default, class
definitions create sealed class instances. To define a class that creates instances that
are not sealed, use the dynamic attribute when declaring the class.
4.17 Slots
A slot is a location inside an instance used to store the value of a variable property. A
slot is allocated for each variable declaration.
4.18 Trait
A trait is a fixed property shared by all instances of the same type. The collection of
traits defines the invariants of the object's type. For this reason, use the traits object to
describe the type of an object. Traits are declared in the definition of the class used to create an object.
class A
{
var x
function m() { }
function get y() { return 10 }
function set y(v) { }
}
Each member of this class definition causes a trait to be added to the traits object for
instances of A. When an instance is created by class A, the resulting object has the properties x, m, and y, implemented by traits for var x, function m, function get y and
functionset y.
Traits express the type of an instance. All traits are copied down to the derived traits objects. All traits must be implemented. Interface members are abstract and so their
traits must be implemented in any class that inherits them.
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5 Names
A name consists of a string and a namespace. Names are introduced into a particular scope by a definition. Those definitions are referred to by names that result from
expressions.
The qualified forms result in a single name consisting of the given qualifier and identifier. The unqualified forms result in a set of names consisting of strings qualified
by the open namespaces.
The visibility of an identifier is controlled by the set of open namespaces. The set of open namespaces includes all of the implicitly opened namespaces and the namespaces
opened by the user. The implicitly opened namespaces are as follows:
Public namespace
Internal namespace for the current package
Private namespace for the current class
Protected namespaces for the current class
The namespaces opened by the user are controlled by the use namespace directives
that are in scope. For example:
namespace mx = "http://macromedia.com/mx"
use namespace(mx)
o.m()
In this example, the reference to o.m() will involve the names qualified by the
namespace mx as well as the implicitly opened namespaces: public, internal, etc.
The terms namespace and qualifier are used interchangeably when talking about qualified names.
5.1 Definition names
A name introduced by a definition might get its qualifier from one of various sources:
Top-level definitions in a package have the package name as their qualifier
Top-level definitions outside of a package are placed into an anonymous namespace that is unique to the source code file that contains the definitions, which means that
such definitions are visible only within that file.
Interface members have the interface name as their qualifier
Dynamic property names have the public namespace as their qualifier
Definitions inside a class have the internal namespace of the current package as
their qualifier, unless a namespace attribute is specified
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A definition with a namespace attribute has its corresponding namespace as its
qualifier
A definition with an access control attribute has the implicitly defined namespace for
that access specifier as its qualifier
It is an error to introduce a name with an identifier that has already been defined in an open namespace in the same scope, but with a different qualifier.
5.2 Reference names
Reference names result from various forms of expressions. The two main distinctions in
these forms are whether the name is qualified or unqualified, and whether the identifier is a literal identifier or an expression.
The following table shows the kinds of references that include qualified and unqualified,
literal and expression names.
Literal Expression
Unqualified o.id, id o[expr]
Qualified o.q::id, q::id o.q::[expr], q::[expr]
A qualified or unqualified literal identifier is equivalent to the dynamic form with its expression operand replaced by a string literal representing the literal identifier
An unqualified expression reference results in multiple names (called a multiname),
one for every open namespace combined with the string value of the
expression expr
A qualified expression reference results in a qualified name that consists of the value
of the qualifier q combined with the string value of the expression expr
5.3 Name lookup
An expression involving a name results in an internal reference value used by certain
operators to perform actions. To describe name lookup, we distinguish between two
types of references: those that include a base object (object references), and those that do not (lexical references.)
Looking up a reference involves determining its ultimate qualified name (in the case of
unqualified references) and its base object.
5.3.1 Object references
Object references result from expressions involving the dot or bracket operators. They
may be qualified or unqualified. The following table shows various forms of object
references.
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Literal Expression
Unqualified o.id o[expr]
Qualified o.q::id o.q::[expr]
We use the expression form of references to describe the name lookup semantics. However, every literal name can be rewritten as an expression name through the
following steps:
If the expression is an unqualified literal name, then replace the dot operation o.id with a bracket operations of the form o['id']
Otherwise, the expression is a qualified literal name, so replace the operand of the
dot operation with the dot operation o.q::['id']
5.3.1.1 Unqualified object references
The unqualified expression o[expr] is a reference to a property of the value of the expression o that has a name that matches one of the names of the set of names
(multiname) composed in the following way:
Let id be the string value of the expression expr
Let m be an empty set of names
For each namespace q in the set of open namespaces:
Let n be a name with the qualifier q and the identifier id
Add n to the set of names m
Return m
The single name of a multiname reference r is determined by the following steps:
Let t be the least derived type of x that contains at least one of the names in the multiname set m of the reference r
Let m' be the intersection of the set of names m and the property names in t
Let n be the set of names in the most derived type of x and in m'
If n is empty, return the name in m that is qualified by the public namespace
If n contains one name, then return that name
Report an ambiguous reference error
The base object of this reference is the value of the expression o.
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Qualified object references
o.q::[expr]
This is a reference to a property inside the value of o that matches a single name.
Because the qualifier is explicit, the qualified name is straightforward to compute:
Let ns be the value of the expression q
Let id be the string value of the expression expr
Return the qualified name consisting of the namespace ns and the identifier id
The base object of this reference is the value of the expression o.
5.3.2 Lexical references
q::[expr]
q::id
id
Lexical references result from expressions involving a name but no base object.
Whether a lexical reference is qualified or unqualified, with a literal identifier or expression, it results in a search of the scope chain of the lexical environment until
either a match is found or the last scope is searched.
The scope chain might include the following kinds of scopes:
Code inside a with statement will have a with frame as the inner most scope on the scope chain.
Code inside a function definition will have an activation object on its scope chain.
Code inside an instance method will have the instance this object on its scope
chain.
Code inside of a class definition, including in instance and static methods, will have
the class objects of its base classes and the current class on the scope chain. The
inner most class object corresponds to the most derived class, and the outermost class object corresponds to the Object class.
Code everywhere has the global object as the outer most object on its scope chain.
The base object of a lexical reference is computed through the following steps:
Let s be the list of scopes enclosing the reference being evaluated.
Let n be the qualified name or set of qualified names that result from the operation
described in section 5.3.1.1 Unqualified object references.
Search the scopes in s starting from the innermost scope and continuing outwards until a scope is found that contains a property that matches n, or all scopes have
If a match is found, return the scope that contains the matching property.
Report a property not found error.
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6 Types
A type is a set of values. Expressions have known values at runtime, and properties have known types at compile time (as well as at runtime.) The various types of
ActionScript 3.0 can be related graphically as a type lattice where the edges of the
lattice indicate subset relationships.
The following diagram shows the relationships between the main built-in types of the
language:
There are three fundamental program visible types (Null, Object and void). What makes these types fundamental is that their union includes all possible values in the language.
Null includes null, void includes undefined, and Object includes every other value. Null
and void are different because they do not have object-like properties (such as toString, valueOf), and they both have values that represent a missing value.
The type Null includes one value - the value that results of the primary
expression null. The value null is used to represent the idea "no value" in the context of an Object typed reference.
The type void includes one value - the value that is the initial value of the global
property undefined and the result of the unary expression void 0. The value undefined is used to represent the idea "no property" or "no value" in the
context of an untyped reference.
While the need for two types that represent the idea of "no value" seems strange to programmers familiar with statically typed object-oriented languages, the distinction is
useful in ActionScript 3.0 to represent the absence of a property or the absence of a
value of an untyped property versus the absence of a typed property. Here is an example:
dynamic class A {
var x : String
var y
}
var a : A = new A
print(a.x) // null
print(a.y) // undefined
print(a.z) // undefined
a.y = 10
a.z = 20
print(a.y) // 10
print(a.z) // 20
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When dealing with dynamic instances, there is little difference between a property that
doesn't exist and a property with no type and no value. But there is a difference between a property that has a type and one that doesn't. This is one of the reasons for
the existence of both types Null and void.
NOTE In ECMA-262 edition 3, program visible values were instances of one of six unrelated types
(Undefined, Null, Boolean, Number, String and Object). Conversions were provided to translate a
value from one type to another. ActionScript 3.0 provides the same conversions between the
primitive types (void/Undefined, Null, Boolean, String, Number, int and uint).
6.1 Type operators
The language includes two type operators that enable programs to test and manipulate values in terms of a type. These type operators are is and as.
6.1.1 Operator is
The is operator appears in expressions of the form:
v is T
The is operator checks to see if the value on the left is a member of the type on the right. For user-defined types and most built-in types, is returns true if the value is an
instance of a class that is or derives from the type on the right, otherwise it
returns false. For built-in numeric types the result cannot be determined by the class of the value. The implementation must check the actual value to see if it is included in
the value set of the type.
The following table shows the results of using various values and types with the is operator:
Value String Number int uint Boolean Object
{} false false false false false true
"string" true false false false false true
"10" true false false false false true
null false false false false false false
undefined false false false false false false
true false false false false true true
false false false false false true true
0 false true true true false true
1 false true true true false true
-1 false true true false false true
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Value String Number int uint Boolean Object
1.23 false true false false false true
-1.23 false true false false false true
NaN false true false false false true
6.1.2 Operator as
The as operator appears in expressions of the form:
v as T
The purpose of the as operator is to guarantee that a value is of certain type, and, if
not, indicate so by returning the value null.
It is common usage to assign the result of an as expression to a property with the same type in that expression. If the destination type does not include null, the
assignment will convert null to the default value of that destination type (that is, false
for as Boolean and 0 for as Number). This results in loss of information about whether
the original value is included in that type. Programs that need to distinguish between when a value is the default value and an incompatible value must assign the result to a
property of type Object, check for null, and then downcast to the ultimate destination
type.
The steps used to evaluate the as operator are:
Let v be the value of the left operand
Let T be the value of the right operand
If T is not of type Type
Throw a TypeError
If v is of type T
Return the value v
Else
Return the value null
6.2 Type Conversions
A type conversion is the translation of a value to a value that is a member of a specific destination type. When the original value is a member of the destination type, the value
is unchanged. We call this an identity conversion.
Type conversions occur at runtime in various contexts:
6 Types
23
Assignment expressions, argument lists, and return statements
The as operator
Other operators
The result of the conversion depends on the context of the expression that yields the
value to be converted:
var x : T = v Implicit conversion to T
var y : T = v as T v or null
var z : T = v + 10 Conversion according to the rules of the operator
Implicit conversions occur when a value is assigned to a property, passed as an
argument to a function, or returned from a function.
When the destination type is a user-defined type T, an implicit conversion will succeed if
the value is an instance of a class that is T or is derived from T. If an implicit conversion
does not succeed, then a type error is thrown.
When the destination type is a primitive type, the implicit conversion is described by
the corresponding abstract procedure (such astoString() and toNumber().) The
following table shows some implicit conversion results:
Value String Number int uint Boolean Object
{} "[object Object]" NaN 0 0 true {}
"string" "string" NaN 0 0 true "string"
"10" "10" 10 10 10 true "10"
null Null 0 0 0 false null
undefined Null NaN 0 0 false null
true "true" 1 1 1 true true
false "false" 0 0 0 false false
0 "0" 0 0 0 false 0
1 "1" 1 1 1 true 1
-1 "-1" -1 -1 2E+32-1 true -1
1.23 "1.23" 1.23 1 1 true 1.23
-1.23 "-1.23" -1.23 -1 2E+32-1 true -1.23
6 Types
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Value String Number int uint Boolean Object
NaN "NaN" NaN 0 0 false NaN
User-defined types do not have built-in conversion operators, so implicit and explicit
conversions behave the same at runtime. Specifically, if a value is not a member of the
destination type, then no conversion exists, implicit or explicit, and a runtime exception
will result from a cast expression and the default value of the destination type (which is null) will be the result of an as expression.
6.3 Type annotations
A type annotation can be used when declaring a variable, a function parameter, or a function return type to guarantee that the associated value will be a member of a
specific data type. A type annotation is a suffix that takes the form of a colon character
(:) followed by a data type. Examples of type annotations follow:
var num:Number
function foo(str:String) : Boolean {}
In the standard dialect, type mismatches are not reported at compile time. Rather, a
runtime type conversion is attempted and an error is reported if the type conversion
fails. For example, the following example not only compiles in the standard dialect, but also does not cause a runtime error:
var num : Number = "1.23"
trace(num is Number) // output: true
In the strict dialect, type mismatches are reported as compile-time errors. Accordingly,
the previous example does not compile in strict mode because the string
value "1.23" is not a member of the Number data type. In other words, a variable v that uses the following syntax will not compile unless v is a member of data
type T:
var v:T
6.4 Runtime versus compile time type
We sometimes refer to a class or interface that helps to define the structure of a value
as the value's type. What we really mean is that that value is a member of that class or
interface type. This distinction is subtle but important. Since a value might belong to any number of unrelated types, to say that it is of a particular type is misleading.
In dynamically typed languages, expressions don't have types; they have values whose
types may change each time the expression is evaluated.
Statically typed languages make the important simplification of associating a type with
every expression, even if it is a very general one, when it is compiled. In this way, the
suitability of an expression can be checked against its use before it is ever actually run. The cost of this added reliability is the loss of flexibility that comes from not having to
think about the types of values.
6 Types
25
function f( o : Object ) {
var x : Number
x = o // Allowed in the standard dialect
}
f(10) // No problem, x gets set to 10
Other places where the differences between dynamic and static type checking can be
seen are property access, and method invocation.
function f( o : Object ) {
o.g()
return o.x
}
Whereas in a static type system, the binding for a method call or property read would
need to be known at compile-time, the standard dialect always defers that checking
until runtime.
The strict dialect has a hybrid type system. Normally, static type rules are used to
check the compatibility of an expression with its destination type, but there are a few
special cases. For example, when an expression on the right-hand side of an assignment expression consists of a reference to a property with no type, name lookup
is deferred to runtime. When an object reference has a base object that is an instance
of a dynamic class, the reference is checked at runtime. These dynamic typing features
are useful when strict dialect programs are interoperating with dynamic features such as XML objects.
6.5 Untyped versus typed properties
A property without a type annotation or with the wildcard annotation * (as in var x : *) is said to be untyped. Writing to an untyped property will always succeed since an
untyped property can hold any value. Expressions that read from an untyped property
are said to be untyped expressions. Assignment from an untyped expression may or may not succeed at runtime depending on whether its value can be implicitly converted
to the destination type. Nevertheless, in the strict dialect, assignments from untyped
expressions are always type-checked at runtime, as in the standard dialect.
Use untyped properties when you want to store the result of an untyped expression
or undefined as one of the values, or when you want to defer type checking to
runtime.
6.6 Object types
All program-visible types other than void and Null derive from type Object. This means
that all values (except undefined and null) have properties that can be accessed by
object references without the need to be wrapped in an object as they were in ECMA-262 edition 3.
6.7 Class types
A class refers to a type or a value depending on its use.
6 Types
26
class A
{
static var x
var y
prototype var z
}
var a : A // A means type A
a = new A // A means value A
The value is a class object that has the form shown in the drawing above. The class
object is CA. When used as a type, it evaluates to its instance traits (TA). When used in a new expression, the class serves as a factory object with a special method that
creates a new instance (OA), which contains an internal delegate property pointing to
the class object's prototype (P) and an internal traits property pointing to the class object's instance traits (TA).
6.8 Interface types
An interface name can only be used where a type is expected:
interface I{}
var x : I // I means type I
x = new I // Error, I is not a value
6.9 Strict dialect and static types
In the strict dialect, both expressions and properties have types. To be used to compute
the value of a property, the expression must have a static type that is compatible with
the type of the property. One way to think about static types of expressions and values is that the static type is a conservative approximation of the set of values that will
result from that expression.
There are three special cases where static type rules are ignored, possibly allowing runtime errors to occur:
Coercions from an untyped expression to any type
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27
Coercions from any type to type Boolean
Coercions between different numeric types
An explicit cast to a user-defined type is only useful in the strict dialect. This is because
the effect of an explicit cast is to defer type checking until runtime, which is already the
case in the standard dialect. This is not necessarily the case for built-in types that have special conversion behavior.
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28
7 Variables
A variable defines a slot with a name and a type.
A variable declared with the const rather than the var keyword, is read-only outside of
the variable's intializer if it is not an instance variable and outside of the instance
constructor if it is an instance variable. It is a verifier error to assign to a const variable outside of its writable region.
Variables exist in the following objects:
Global object, inside and outside of a package
Class objects
Instance objects
Activation objects
7.1 Variable modifiers
When allowed by the context of the definition, the following attributes modify a variable definition.
Access control namespaces
User defined namespaces
static
prototype
Access control and visibility control namespaces specify the namespace part of the
variables name.
The static attribute may only be used inside a class definition and causes the variable to
become a trait of the class object rather than the instance object.
The prototype attribute may only be used inside a class definition and causes the
variable to be added to the class's prototype object and a get and set accessor to be added to the instance traits of the class. The purpose of the accessor methods is to
simulate the behavior of accessing prototype properties in ECMA-262 edition 3.
7.2 Variable types
All variables can have a type. A type annotation on a variable definition limits the set of
values that can be stored in that variable. A type annotation must be a compile-time
constant expression that evaluates to a class or interface value. The actual value used to represent the type of the variable is the instance traits of the referenced class or
interface.
7 Variables
29
When a value is assigned to a variable, an implicit conversion to the variable's type is
performed on the value. A type error occurs if there is no implicit conversion of that value to the variable's type. In the strict dialect, such errors are verification errors; in
the standard dialect, type errors are runtime errors.
If no type is specified, or if the wildcard annotation * is specified (as in var x : *), the variable is said to be untyped.
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30
8 Functions
A function is a callable object. In general, functions consist of a block of code, a set of traits, and a list of scopes. Instance methods are functions that also consist of a
receiver object to which this references are bound.
8.1 Function modifiers
When allowed by the context of the definition, the following attributes modify a function definition:
Access control namespaces
User defined namespaces
static
final
override
native
Access control and visibility control namespaces specify the namespace part of the
function name.
The static attribute may only be used inside a class definition and causes the function to become a trait of the class object rather than the instance object.
The final attribute may only be used on a non-static function definition inside a class.
A function modified by final cannot be overridden.
The override attribute may only be used on a non-static function definition inside a
class. A function modified by override will override a method with the same name and
signature as a non-final method of a base class.
The native attribute may be used to indicate that the function is implemented in an implementation-defined way. The compiler should generate native stubs for functions
that have this attribute.
8.2 Function signatures
A function signature includes the number and types of its parameters and its result
type. Like variable type annotations, the types of a function signature affect the implicit
conversion of argument and return values when calling to and returning from a function. Function signatures are also used to match inherited methods to methods in a
derived class.
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8.3 Function objects
Global and nested functions can be used as constructors in instantiation expressions, as
shown in the following example:
function A() { this.x = 10 }
var o = new A
trace(o.x) // traces 10
Function objects have a property named prototype whose value is used to initialize the
intrinsic delegate property of the objects it creates. The prototype property has a default value of a new instance of the class Object. Building on the previous example:
function A() { this.x = 10 }
function B() {}
B.prototype = new A
var o = new B
trace(o.x) // traces 10
The value of o is an instance of B, which delegates to an instance of A, which has a
property named x with value of 10.
Constructor methods inside of a class are also used to create objects. But, unlike
constructor functions, constructor methods create objects with a set of fixed properties
(traits) associated with its class and a delegate that is also an instance of its class.
class A {
var x
function A() { this.x = 10 }
}
var o = new A
trace(o.x) // traces 10
There are some subtle differences between the preceding example and the one
involving a function constructor:
x is a fixed property of each instance of A rather than a dynamic property.
A.prototype is an instance of A rather than an instance of Object.
The expression A(expr) does not call the function A defined in class A. It results in
an explicit conversion of the value of expr to the type A.
Class methods are functions that are defined with the static attribute inside of a class definition. A class method cannot be used as a constructor and does not define
the this reference. Class methods are in the scope of the class object in which they are
defined.
Instance methods are functions that are defined without the static attribute and inside
a class definition. Instance methods are associated with an instance of the class in
which they are defined. Instance methods can override or implement inherited class or
interface methods and always have a value bound to this.
The value of this in an instance method is the value of the instance the method
belongs to. When an instance method is extracted from an object, a bound method is
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32
created to bind the value of this to that host object. Assignment of the bound method
to a property of another object does not affect the binding of this. For example:
class A {
var x
function A() { this.x = 10 }
function m() { trace(this.x) }
}
var a = new A()
var o = { x : 20 }
o.m = a.m
o.m() // traces 10
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9 Classes
A class is a type, a constructor of objects of that type, and a singleton object for sharing state and behavior. It is used as a constructor to create like instances. It is
used as a type to constrain the value of properties. It is used as a singleton object to
contain shared properties.
Classes are introduced with class definitions. A class definition can directly extend one
other class definition and implement multiple interface definitions. The language does
not support the concept of abstract classes and so a class must implement every interface method it inherits.
9.1 Class modifiers
Class definitions may be modified by these attributes
dynamic Allow properties to be added to instances at runtime
final Must not be extended by another class
internal Visible to references inside the current package (default)
public Visible to references everywhere
The default modifiers for a class definition are internal, non-dynamic, and non-final.
9.2 Class objects
Class objects have the basic structure shown in the following illustration:
The illustration shows the shape of the class object that results from the following
simple class definition:
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9.2.1 Prototypes
Every object has a prototype object that is used to match references at runtime. This
prototype is called the delegate of the object. Delegation is a simple way to add shared
properties to a group of related objects at runtime.
Prototype objects are always instances of the dynamic class Object and therefore can
always be extended by the addition of dynamic properties. Unlike with function closures
that have a prototype property that is a variable and can be reset to another object,
classes have a prototype that is read-only and so always point to the same object.
9.2.2 Traits
Properties of a class definition are represented as traits of the class object and its
instances. Think of a trait as a fixed property that is shared by all instances of a type. Class objects (CA) are special in that they are a single instance with an internal type
with a corresponding set of traits (TCA). The internal type of a class object describes
the static properties of the class definition. The instance traits (TA) are shared by all instances created by the class object. They correspond to the instance properties of the
class definition.
class A
{
static var x
var y
}
In this example, the definition for x contributes a trait to the class traits (TCA), and the definition of y contributes a trait to the instance traits (TA).
9.2.3 Methods
Each function definition inside a class definition results in a method inside the resulting
class object or its instances. Two special methods are implicitly defined for each class: a class initializer; and an instance initializer. Code outside a function definition gets
placed in the class initializer, which is called when the class object is created. Instance
variable initializers are placed in the instance initializer method, which is called when an instance of the class is created and before the user-defined constructor is executed.
9.2.4 Slots
Traits introduced by variable definitions describe a property that holds a value unique to
each instance. Therefore, each object has a fixed array of slots that store those values, one for each variable trait. This is true of class objects as well as instance objects.
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9.2.5 Instances
All instances (OA) created by a class object (CA) will be given a traits (TA) and delegate
(PA) object, as represented in this drawing
9.2.6 Inheritance
Each class inherits the instance traits of its base class. These traits are effectively
copied down to the instance traits of the derived class. Classes that don't declare an
explicit base class inherit the built-in Object class.
A class may also inherit the instance traits of one or more interfaces. Interface traits
are abstract and so must be implemented by any class that inherits them.
Unlike static properties in other object-oriented languages, such as Java, static properties in ActionScript are not inherited, but they are in scope in the static and
instance methods of the derived class.
9.2.7 Scopes
Static properties are in scope of bodies of static and instance methods of the same class. Instance properties are in scope of the bodies of the instance methods. Instance
properties shadow static properties with the same name. Static properties of base
classes are in scope of static and instance methods of a class.
class A
{
static var ax
}
class B extends A
{
static var bx
}
class C extends B
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{
static var cx
var ix
function m()
{
var mx
gx = 10
ax = 20
bx = 30
cx = 40
mx = 50
}
}
var gx
o = new C
o.m()
Scopes:
{ mx } - activation scope
{ ix } - instance scope
{ cx } - static scope C
{ bx } - static scope B
{ ax } - static scope A
{ gx } - global scope
9.3 Class property attributes
Class properties may be modified by the following attributes
static Defines a property of the class object
private Visible to references inside the current class
internal (default) Visible to references inside the current package
protected Visible to references inside instances of the current class and derived classes
public Visible to references everywhere
AttributeExpression Namespace value is the qualifier for the name of the definition
It is a syntax error to use any other attribute on a class property, unless otherwise specified in the section describing the specific type of property.
9.3.1 Static attribute
The static attribute means the current definition defines a property of the class object.
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9.3.2 Prototype attribute
The prototype attribute was not implemented in ActionScript 3.0.
9.3.3 Access control namespace attributes
Each access control attribute (private, internal, protected, and public) refers to a
namespace value with a unique, private namespace name. Access control is provided by the fact that code outside of the attribute's access domain has no way to refer to
that namespace value.
9.3.4 User-defined namespace attributes
The value of an attribute expression that evaluates to a compile-time constant
namespace is used as the qualifier of the definition's name.
namespace ns
class A
{
ns var x
}
In the preceding example, the name of the definition of x is qualified by the namespace ns. Note the following rules:
Only one namespace attribute may be used per definition.
Namespace attributes may not be used with an access control attribute.
9.4 Class body
A class body may contain variable definitions, namespace definitions, function definitions, and statements:
class A
{
static var x
static function f() {}
var y
function g() {}
trace("class loaded")
}
Definitions result in class or instance traits depending on whether the static attribute occurs in their definition.
Statements and initializers of static variables are added to the static initializer method of the class. The static initializer is called once, when the class is defined at
runtime. The static initializer can be used to initialize variables of the class object
and to invoke methods that are external to the current class.
9 Classes
38
Initializers of instance variables are added to the instance initializer method.
The scope chain of methods contained by the class body includes the class object, the base class objects (from most derived the least derived), and the global object.
Note that it is not an error to define a class and instance property with the same name,
as in the following example:
class A {
static var x
var x
}
It is also not an error to define a class property with the same name as a visible class
property in a base class:
class A {
static var x
}
class B extends A {
static var x
}
9.5 Class variables
Class variables are defined using the var or const keywords.
class A
{
var x
const k = 10
}
The meaning of var and const follow from the general meaning described in the
sections 7 Variables and 1.4 Variables.
var May be written to multiple times
const May be written to only once
const variable properties can be written to only once. The compiler uses a specific data
flow analysis to determine if a const variable has been written to at the point of an assignment to that variable. Informally, the effect of this algorithm can be seen in the
following error cases:
It is an error to assign to a const instance or static variable in a statement that is outside of the instance or static initializer, respectively.
It is an error to assign to a const variable more than once in a sequence of statements with no control flow branches.
It is an error to assign to a const variable in more than one parallel control flow
branch if the branch conditions are not compile-time constant expressions, or if the
value of those branch conditions allow for one or more of those branches to be
executed more than once.
The default value of a class or instance variable is the value of undefined coerced to the
type of the variable.
9.5.1 Static variables
Variables declared with the static attribute add a slot trait to the class traits and a slot to the class object. Because there is only one class object per class, there is also only
one slot per static variable. Static variables, like static methods, are not inherited, but
are accessible from within the body of the class definition and through an explicit reference to the defining class's name. Static variables are in scope for all static and
instance methods of the defining class and classes that inherit the defining class.
Static const variables must either have an initializer or be definitely unassigned before being set in the static initializer method.
NOTE Unlike in Java and C#, static variables are not inherited by derived classes and so cannot be
referenced through derived class objects.
9.5.2 Instance variables
Variables declared without the static attribute add a slot trait to the instance traits of the class and a slot to each instance of the class. Instance variables are always final
and must not be overridden or hidden by a derived class.
As with all class properties, the default qualifier for the variable is the internal namespace. Other qualifiers can be specified by other namespace attributes. Both
instance and class variables are implicitly final. Any attempt to hide or override one in a
derived class will result in a verification error.
9.6 Class methods
A method is a function associated with a specific object. Unlike a function closure, a
method is not a value and cannot be used apart from the instance to which it is bound. The value of this inside a method is always the base object used to refer to the
method, and always has the type of the class that implements the method, or
subclasses of that class.
9.6.1 Constructor methods
A function declared with the same identifier as the class it is defined in adds a
constructor method to the class object. The constructor is called when a new instance
of that class is created. A constructor may refer to the instance variables of the class that defines it.
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40
class A
{
function A() {}
}
A constructor is public by default and may be defined with the public namespace or with
no namespace attribute. If no constructor is defined by a class definition, a default constructor is defined implicitly. No more than one constructor can be defined for a
class.
If the body of a constructor contains a SuperStatement, that statement must occur before the first reference to this or super, and before any return or throw statement.
If a call to the super constructor is not explicit, one will be inserted before the first
statement in the constructor body. Note the following errors:
It is a syntax error to call the super constructor more than once.
It is a syntax error to specify a return statement with an expression.
It is a syntax error to specify a result type of a constructor.
NOTE That there is no way to directly call the constructor of an indirect base class is intentional because it
might lead to brittle or insecure programs.
9.6.2 Static methods
Functions declared with the static attribute add a method trait to the class object
traits. Static variables are in scope of a static method.
It is an error for the this or super expression to appear in the body of a static method.
NOTE Unlike in Java and C#, static variables are not inherited by derived classes and so cannot be
referenced through derived class objects.
9.6.3 Instance methods
Functions declared without the static attribute add a method trait to the instance
traits of a class object. Static and instance variables are in scope of an instance method. The value of this inside an instance method is the instance the method is
bound to.
class A
{
function m() { return this }
}
var a = new A
trace(a==a.m()) // trace true, this is the object 'm' is called on
In addition to the attributes defined for all class properties, the following attributes may be used on instance methods
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41
final May not be overridden
override Must override an inherited method
The override attribute helps to avoid unintentional overriding of base class methods. It
is a verifier error to use the overrideattribute on a function definition that does not
override an inherited method. It is a verifier error to override an inherited method that
is declared final. It is an error to define a method without the override attribute if the name matches the name of an inherited method.
The prototype attribute allows the addition of a fixed property to the prototype object,
but not to the instance. Instance methods defined with the prototype attribute have function values that are compatible with ECMA-262 edition 3 prototype functions.
class A
{
prototype var f = function() { return this }
}
var a = new A
dynamic class B {}
var b = new B
b.f = a.f
b.f() // traces "[object B]"
The instance of B becomes the value of this.
9.6.4 Accessor methods
A method defined with the get or set keyword adds a get or set method trait to the
instance or static traits of the defining class object. Accessor methods are called when
the name of the accessor is used in a reference that reads or writes the value of that name.
class A
{
private var _x
function get x() { return _x }
function set x(v) { _x = v }
}
var a = new A
a.x = 10 // calls set accessor of A
trace(a.x) // traces 10, calls get accessor of A
Accessor methods are very similar in definition to regular methods. The differences are
expressed by the following error conditions:
Get methods must specify no parameters.
Set methods must specify just one parameter.
Get methods must return a value.
Set methods have a result type void by default.
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Set methods must not specify a result type other than void.
Get methods must not specify the result type void.
If both a get and set method is defined with the same name, the parameter type of
the set method and the result type of the get method must match.
NOTE Accessors may only be defined at the top level of a class. They must not be nested inside another
method or defined outside of a class.
9.6.5 Inheriting instance methods
Instance methods are inherited by copying their instance traits down to the instance
traits of the derived class.
9.6.5.1 Overriding instance methods
Methods inherited from a class may be overridden in the derived class if the overriding method is given the override attribute and if its name, number and type of parameters,
and return type match exactly. It is an error to attempt to override a method with a
method that has the same name, but does not have the same number of parameters or parameters of different types or different return type.
9.6.5.2 Implementing interface methods
Methods inherited from an interface must be implemented by a method with a name
and signature that matches the inherited method. Interface methods are implemented by an instance method declared with the public attribute.
A method that has the public attribute implements all inherited interface methods with
a matching identifier.
interface I
{
function m()
}
interface J
{
function m()
}
class A implements I,J
{
public function m() { trace("A.m") }
}
In this example, the definition of m in class A satisfies both interfaces I and J.
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9.6.6 Bound methods
Although a method is not a value by itself, it can be converted to a first class value
called a bound method, through extraction. A bound method maintains the binding
between a method and its instance. The user-visible type of a bound method is Function.
class A
{
function m() { return this }
}
var a = new A
var mc : Function = a.m // create a bound method from m and a
trace(a==mc()) // trace true, mc remembers its this
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44
10 Interfaces
Interfaces provide a way for programs to express contracts between the producers and consumers of objects. These contracts are type safe, easy to understand, and efficient
to implement. Programs should not have to pay a significant performance penalty for
using interfaces.
An interface is a type whose methods must be defined by every class that claims to
implement it. Multiple interfaces can be inherited by another interface through
the extends clause or by a class through the implements clause. Instances of a class that implements an interface belong to the type represented by the interface. Interface
definitions must only contain function definitions, which may include get and set
methods.
Interface methods are not public by default, but are added to the public namespace by the implementing method definition.
10.1 Interface types
An interface definition introduces a type into the current scope. The interface type is described by a set of abstract method traits and a list of interfaces that it extends. This
set of abstract traits must be fully implemented by any class that inherits the interface.
An interface name refers to the interface type when it is used in a type annotation or an inheritance clause of a class or interface definition.
interface I {}
class A implements I {} // I refers to type I
var x : I = new A // In each of these uses too
trace( x is I )
var y : I = x as I
When a reference is bound to an interface at compile-time, the value of that reference
is always the compile-time interface value, even if the interface definition would be shadowed by another property at runtime. This is shown in the following example:
interface T {}
class A implements T {}
class B {}
function f() {
var T = B
var x = new A
trace(x is T) // T refers to interface T, not var T, traces true
}
In this example, T in the is expression refers to the outer interface T, not the inner
var T.
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10.2 Interface methods
Classes that implement an interface method must use the public attribute to
implement all interface methods that have the same identifier name. The following
example shows a class that implements two inherited interfaces with public qualified methods.
interface I
{
function f()
}
interface J
{
function g()
}
class A implements I
{
public function f() {}
public function g() {}
}
10.2.1 Visibility of interface methods
Interface methods are visible when referenced through a property of the corresponding
interface type or through a reference to the implementing class or subclass.
var a : A = new A
a.f() // okay, f is visible through an A as {public}::f
a.g() // okay, g is visible through an A as {public}::g
var i : I = b
i.f() // okay, f is still visible through an I as {I}::f
i.g() // error, g is not visible through an I as {I}::g
References through an object with an interface type are multinames that contain only
the names qualified by the interface namespace and its super interface namespaces.
This means that the names in the open namespaces (including public) will not be visible through a reference with an interface-typed base object. The motivation for this
behavior is to express the idea of the interface as a contract between the producer and
consumer of an object, with the contract specified by the names in the interface namespace alone.
If the compile-time type of the base object is not an interface type, an unqualified
reference will use the currently open namespaces (which includes public) to create a multiname in the normal way. Again, ambiguous references can be explicitly qualified
with the interface name to avoid conflicts.
10.2.2 Inheritance of interface methods
The rules for implementing an inherited interface method are the same as the rules for overriding an inherited class method. Specifically, the name of the method, number and
type of the parameters, and return type must match exactly.
10 Interfaces
46
It is a verification error if a class implements an interface method with a method whose
name matches, but the parameter count or types or return type do not match. It is a verifier error if a class inherits an interface method that it does not implement.
10.3 Interface example
The following example shows how interfaces are defined and used.
An implementing class must use public as an attribute to make the method implement all interface methods with a matching identifier.
The static type of the base object of a reference controls which interface names are
open in that reference if that type is an interface type.
11 Packages
47
11 Packages
A package definition introduces a top-level namespace, suitable for organizing collections of type definitions into APIs.
Unlike ordinary namespaces (hereafter referred to simply as namespaces), a package is
a pure compile-time construct. A package directive qualifies the names of properties defined inside of it at compile time; references to a package's member definitions are
given fully qualified names at compile time.
package mx.core
{
class UIObject extends ...
{
}
.
.
.
}
In this example, the fully qualified name for UIObject is mx.core.UIObject. An unqualified reference to UIObject will be fully qualified as mx.core.UIObject by the
compiler.
Package definitions may be discontinuous; the definition of a package may be spread
over multiple package definitions, possibly in multiple source files.
The semantics of loading packages is outside of the language definition. The compiler
and virtual machine will have access to the package definitions in files that have been
loaded by the embedding tool or runtime.
11.1 Package namespace
The namespace name (the string used for equality comparision) of a package is the
sequence of characters of its name. For example, the package in:
package mx.core {
.
.
.
}
is given the namespace name "mx.core".
Package names are used to:
Qualify the names of top-level definitions in a package
Qualify the names of references to those definitions
Import names into other packages.
11 Packages
48
package acme.core
{
public class Widget { } // qualifies Widget
}
import acme.core.* // make visible all names in acme.core
var widget : acme.core.Widget // distinguishes a reference to Widget
Packages exist only at compile time. The static existence of packages allows us to give
them certain properties that would not be possible if they could be manipulated at runtime. In particular:
Package names may have embedded dots.
Fully qualified package references may and must be expressed using the dot
operator rather than the usual :: syntax for qualified names
But because there is no runtime value for a package name, packages cannot be aliased
or otherwise used in an expression that uses a runtime value.
When encountered in a valid context by the compiler, the meaning of a package name becomes fixed; any interpretation at runtime is no longer possible.
For this reason, a package name always shadows locally defined names, independent of
the scope chain, when that package name is used on the left hand side of a dot
operator.
package p
{
public var x = 10
}
import p.x
function f()
{
var p = { x : 20 }
trace(p.x) // traces 10
}
f()
The following strict errors affect packages:
It is a strict error to import a package that cannot be found.
It is a strict error to reference a package property that cannot be found in an
imported package.
11.2 Package members
Definitions with the public attribute inside a package definition are implicitly qualified
by the package namespace. Every kind of definition except for package definitions may appear directly inside a package definition, including variable, function, namespace,
class, and interface definitions.
11 Packages
49
11.2.1 Package property attributes
The visibility of a name defined inside of a package is controlled by the attributes that
appear in that definition. Allowed attributes include the following:
public Qualified by the package namespace
internal Qualified by the internal namespace for the current package [default]
It is a syntax error for more than one of these attributes to appear in a definition.
11.3 Package import
The names of package members are made visible inside an external scope with an import directive. For example, the following code makes all public names defined in
the package mx.core visible inside any scope that contains this directive:
import mx.core.*
Individual names can be imported using an import directive with the fully qualified name to be imported. For example, the following code has the effect of making the
class mx.core.Image, but no other names defined inside package mx.core, visible to an
unqualified reference.
import mx.core.Image
References to package members are fully qualified using the dot operator. When the meaning of a simple name is ambiguous, a fully qualified name can be used to indicate
the intended binding. For example:
import mx.core.*
import player.core.*
new Image // error, mx.core.Image or player.core.Image?
new player.core.Image // okay
Visibility of package members outside of a package is controlled by access control namespaces. The default namespace of a package member is package internal. For
example:
package acme.core
{
public class Widget { }
class WidgetImpl {} // default namespace is internal
}
import acme.core.*
new WidgetImpl // error, cannot find WidgetImpl
new Widget // okay, public names are always visible
In this example, class WidgetImpl is in the internal package namespace for package acme.core. This namespace is always open inside of any definition of
package acme.core, and never open or accessible outside of a definition of acme.core.
11 Packages
50
11.3.1 Single name aliases
A name alias can be provided for single name import directives to avoid ambiguity of
unqualified references, as shown in the following code:
package acme.core
{
public class Widget { }
}
package mx.core
{
public class Widget { }
}
import AcmeWidget = acme.core.Widget
import MxWidget = mx.core.Widget
new AcmeWidget
new MxWidget
When an alias is specified, the original fully qualified name can be used to refer to the
imported definition. It is also possible to use the original unqualified name as long as the resulting reference is not ambiguous.
11.4 Unnamed package
The unnamed package is defined by a package definition with no name specified, as shown in the following code:
package
{
}
The unnamed package is implicitly imported by all other packages and global code
outside of any package. This makes it convenient for casual sharing of definitions between programs by making public definitions in the unnamed package always visible.
12 Namespaces
51
12 Namespaces
Namespaces are used to qualify names. ECMAScript for XML (E4X) introduced the idea of explicitly qualifying names to reference properties of an XML object. XML
namespaces allow markup with various meanings, but potentially conflicting names, to
be intermixed in a single use. Packages in ActionScript 3.0 provide such a capability. XML namespaces also allow names to be individually qualified to create sub-
vocabularies relating to concerns secondary to the main purpose of the markup.
Namespaces in ActionScript 3.0 provide this capability, that is, controlling the visibility of names independent of the structure of the program. This is useful for giving trusted
code special access privileges and for distinguishing the meaning of a name between
versions and uses.
12.1 Namespace values
Namespace definitions introduce a constant fixed property of type Namespace into the
defining scope. The property is initialized to an implicit or explicit value. Regardless of
how it is initialized, a namespace value consists of a namespace name used for equality comparison.
The following example shows the definition of several namespaces:
N1 is given an anonymous namespace name. N2 is an alias of N1. N3 is given a namespace with the namespace name of'http://www.ecma-
international.org/namespace'. When created by a namespace definition, the prefix
of a namespace is initialized to the value undefined.
The set of attributes that may be used on a namespace definition is the same as the set
that can be used on a variable definition.
12.2 Namespaces as attributes
When used as an attribute of a definition, a namespace specifies the namespace qualifier of that definition's name:
namespace N1
namespace N2
N1 var x : int = 10
N2 var x : String = "hello"
Here, two distinct variables are defined--one with the qualified name N1::x and the
other with the qualified name N2::x. Referencing code can refer to one or the other of these names by explicitly qualifying references to x or by adding one or the other
namespace to the set of open namespaces.
It is an error to use a user-defined namespace as an attribute except in the top-level of a class definition.
12 Namespaces
52
12.3 Namespaces as qualifiers
References to a name qualified by a namespace can be explicitly qualified by that
namespace:
namespace N1
namespace N2
N1 var x : int = 10
N2 var x : String = "hello"
trace(N1::x)
In this case, the qualification is necessary because an unqualified reference to x would
not match any visible definition of x, and would therefore result in a runtime exception.
12.4 Open namespaces
The set of open namespaces determines the visibility of unqualified references. If the
qualifier of a name is not in the set of open namespaces, it will not be visible to an unqualified reference. Namespaces are added to the list of open namespaces by the use
namespace directive. Building on the previous example, the namespace N1 is added to
the set of open namespaces:
namespace N1
namespace N2
N1 var x : int = 10
N2 var x : String = "hello"
use namespace N1
trace(x) // trace 10
The unqualified reference to x matches any name that has the identifier x and is
qualified by one of the open namespaces, in this caseN1::x.
It is a runtime error for more than one name to match an unqualified reference.
The set of open namespaces includes any namespace that is explicitly used in that
block or an outer nested block, as well as the public, internal, protected, and private
namespaces that are implicitly open in various contexts.
Bindings of explicitly used namespaces are preferred over names in the public
namespace. This allows a public name to be given an open user-defined namespace
without making unqualified references ambiguous:
namespace N1
N1 var x : int = 10
public var x : String = "hello"
use namespace N1
trace(x) // okay, matches N1::x, even though public::x is also visible
12 Namespaces
53
12.5 Namespace examples
12.5.1 Access control
class A {
private namespace Key
private var friends = [ B ]
function beMyFriend( suitor ) {
for each( friend in friends )
{
if( suitor is friend ) return Key
}
return null
}
Key function makeMyDay()
{
trace("making my day")
}
}
class B {
function befriendAnA(a:A) {
var key : Namespace = a.beMyFriend(this)
if( key != null )
{
a.key::makeMyDay()
}
}
}
12.5.2 Version control
package p {
public namespace V2
public class A {
public function m() {}
V2 function m() {}
}
}
import p.v1
import p.v2
import p.A
// version 1
class B extends A
{
public function m() {}
}
// version 2
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54
class B extends A
{
public function m() {}
V2 function m() {}
}
use namespace p.V2 // open p.V2, prefer it over public
var a : A = new B
a.m()
12.5.3 Vocabulary control
Namespace definitions allow multiple vocabularies to be defined in a single class. This is a kind of polymorphism that is independent of the class abstraction. It is useful when
you have common functionality that has a more than one public interface. You could
use subclasses to express the overridden behavior, but if there is more than one
vocabulary that needs to be mixed in, the number of combinations quickly explodes.
package p {
public namespace French
public namespace Vegan
public class Person {
public function sayIt() { /* say it in English */ }
French function sayIt() { /* say it in French */ }
public function eatIt() { /* eat steak */ }
Vegan function eatIt() { /* eat vegan */
}
}
import p.*
var person = new Person()
{
use namespace French
use namespace Vegan
person.sayIt() // speak French
person.eatIt() // eat vegan
}
{
person.sayIt() // speak English
person.eatIt() // eat meat
}
13 Lexical Structure
55
13 Lexical Structure
13.1 Lexical
Lexical keywords are removed from the available program namespace during scanning.
It is a syntax error to use any of these names except as indicated by the grammar.
Syntactic keywords appear to the lexical scanner as identifier tokens, but are given
special meaning in certain contexts by the parser.
The following list contains all keywords:
as break case catch class const continue default delete do else extends
false finally for function if implements import in instanceof interface
internal is native new null package private protected public return super
switch this throw to true try typeof use var void while with
The following list contains all identifiers that are syntactic keywords:
each get set namespace include dynamic final native override static
13.2 Syntactic
Identifiers with special meaning become keywords in certain syntactic contexts:
In a for-each-in statement between the 'for' token and the '(' token:
each
In a function definition between the 'function' token and an identifier token:
get set
As the first word of a directive:
namespace include
In an attribute list or wherever an attribute list can be used:
dynamic final native override static
It is a syntax error to use a syntactic keyword in a context where it is treated as a
keyword:
namespace = "hello"
namespace()
In these cases, the grammar requires an identifier after the namespace keyword.
14 Expressions
56
14 Expressions
The syntax in this section contains the following superscript and subscript symbols:
The opt subscript is attached to symbols that are optional.
The allowIn superscript is attached to nonterminals that can be defined with a
production rule that contains the in operator.
The noIn superscript is attached to nonterminals with definitions that do not include a production rule that contains the inoperator. This superscript is necessary to
avoid conflicts between the in operator as part of a relational expression and
the inoperator as part of a for statement.
The β superscript is shorthand notation that denotes a nonterminal with a pair of
definitions: an allowIn version and a noInversion.
14.1 Identifiers
Identifiers may be either simple identifiers or qualified identifiers. Qualified identifiers
result in a single name consisting of a namespace and a string. The string is specified
by an expression or a literal identifier. The namespace is specified by an expression
that precedes the :: punctuator. Simple identifiers result in one or more names that consist of the identifier string and each of the namespaces open in the scope of the
expression. The resulting name value(s) are used to construct a Reference value
Let name be the result of evaluating NonAttributeQualifiedIdentifier
Return name
14 Expressions
62
14.2 Primary expressions
Syntax
PrimaryExpression
null
true
false
Number
String
this
RegularExpression
QualifiedIdentifier
XMLInitializer
ReservedNamespace
ParenListExpression
ArrayInitialiser
ObjectInitialiser
FunctionExpression
A PrimaryExpression can be used wherever a FullPostfixExpression or a FullNewSubExpression can be used. This includes object creation, property access, and
function invocation expressions.
Verifition
PrimaryExpression : null
Return the type Null
PrimaryExpression : true
PrimaryExpression : false
Return the type Boolean
PrimaryExpression : Number
Return the type Number
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63
PrimaryExpression : String
Return the type String
PrimaryExpression : RegularExpression
Return the type RegExp
PrimaryExpression : QualifiedIdentifier
Return the result of verifying QualifiedIdentifier
PrimaryExpression : XMLInitialiser
PrimaryExpression : ReservedNamespace
PrimaryExpression : ParenListExpression
PrimaryExpression : ArrayInitialiser
PrimaryExpression : ObjectInitialiser
PrimaryExpression : FunctionExpression
Return the result of verifying the non-terminal symbol on the right-hand side of the production
PrimaryExpression : this
Let frame be the immediately enclosing ParameterFrame
If frame is none
Throw a VerifyError
Return the result of typeOfThis(frame)
Evaluation
PrimaryExpression : null
Return the value null
PrimaryExpression : true
Return the value true
PrimaryExpression : false
Return the value false
PrimaryExpression : Number
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64
Return the Number value produced by lexical analysis of Number
PrimaryExpression : String
Return the String value produced by lexical analysis of String
PrimaryExpression : this
Let frame be the immediately enclosing ParameterFrame
Return the value of this associated with frame
PrimaryExpression : RegularExpression
Return the RegExp result of evaluating the expression produced by lexical analysis of RegularExpression
PrimaryExpression : QualifiedIdentifier
Let name be the result of evaluating QualifiedIdentifier
Let ref be an instance Reference(null,name,null)
Return ref
14.3 Reserved namespace expressions
Syntax
ReservedNamespace
public
private
protected
internal
Verification
ReservedNamespace : public
Return the value of type Namespace
ReservedNamespace : private
ReservedNamespace : protected
If ReservedNamespace is not enclosed in a ClassDefinition
Throw a VerifyError
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65
Return the value of type Namespace
ReservedNamespace : internal
If ReservedNamespace is not enclosed in a PackageDefinition
Throw a VerifyError
Return the value of type Namespace
Evaluation
ReservedNamespace : public
Return the public namespace
ReservedNamespace : private
Return the private namespace of the enclosing class
ReservedNamespace : protected
Return the protected namespace of the enclosing class
ReservedNamespace : internal
Return the internal namespace of the enclosing package
14.4 Parenthesized expressions
Syntax
ParenExpression
( AssignmentExpressionallowIn
)
ParenListExpression
ParenExpression
( ListExpressionallowIn
, AssignmentExpressionallowIn
)
Verification
ParenExpression : ( AssignmentExpressionallowIn )
Return the result of verifying AssignmentExpression
Let ref be the result of evaluating AssignmentExpression
Let val be the value of referenceRead(ref)
Call objectWrite(obj,name,val)
FieldName : NonAttributeQualifiedIdentifier
Return the result of evaluating NonAttributeQualifiedIdentifier
FieldName : String
Return the value of String
FieldName : Number
Let num be the value of Number
Return the result of calling String(num)
14.7 Array initialiser
An array initialiser is an expression describing the initialisation of an Array object, written in a form of a literal. It is a list of zero or more expressions, each of which
represents an array element, enclosed in square brackets. The elements need not be
literals; they are evaluated each time the array initialiser is evaluated.
Array elements may be elided at the beginning, middle or end of the element list.
Whenever a comma in the element list is not preceded by an AssignmentExpression
(such as a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements.
Elided array elements are not defined.
Syntax
ArrayInitialiser
[ ElementList ]
14 Expressions
70
ElementList
«empty»
LiteralElement
, ElementList
LiteralElement , ElementList
LiteralElement
AssignmentExpressionallowIn
Verification
An ArrayInitialiser is verified by verifying all non-terminals on the right-hand side of each production. The result of verifying an ArrayInitialiser is the type Array.
Evaluation
ArrayInitialiser expressions are evaluated as described in ECMA-262 edition 3.
14.8 XML initialisers
An XML initialiser is an expression describing the initialisation of an XML object, written
in a form of a literal. It may specify an XML element, an XML comment, an XML PI, or a
CDATA section using ordinary XML syntax. For XML elements, it provides the name,
See the ECMAScript for XML (E4X) specification (ECMA-357 edition 2) for definitions of XMLMarkup, XMLText, XMLName, XMLWhitespace and XMLAttributeValue.
Verification
An XMLInitialiser is verified by verifying all non-terminals on the right hand side of each
production. The result of verifying an XMLInitialiser is the type XML.
Evaluation
XMLInitialiser expressions are evaluated as described in ECMA-357 edition 2.
14 Expressions
72
14.9 Super expression
SuperExpression limits the binding of a reference to a property of the base class of the
current method. The value of the operand must be an instance of the current class. If
Arguments is specified, its value is used as the base object of the limited reference. If no Arguments is specified, the value of this is used as the base object.
Syntax
SuperExpression
super
super Arguments
SuperExpression may be used before a PropertyOperator in either
a FullPostfixExpression or a FullNewSubexpression.
super.f(a,b,c)
super(o).f(a,b,c)
Verification
SuperExpression : super
SuperExpression : super Arguments
Let frame be the immediately enclosing ParameterFrame
If frame is none
Throw a VerificationError
Let type be the result of typeOfThis(frame)
Let limit be type.super
If Arguments is specified and not empty
Let obj be the result verifying Arguments
Call verifyType(obj,limit)
Return the type limit
Evaluation
SuperExpression : super
SuperExpression : super Arguments
Let frame be the immediately enclosing ParameterFrame
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73
Let this be the value of frame.this
Let type be the value of this.type
Let limit be type.super
If Arguments is empty or not specified
Let obj be the value of this
Else
Let obj be the result of evaluating Arguments
If obj.type is not a subtype of limit, then throw a TypeError
Let obj be a new instance LimitedBase(obj,limit)
Compatibility
ActionScript 2.0 supports only the first form of SuperExpression.
super.f(a,b,c)
This is equivalent to the following ActionScript 2.0 code:
Let ref be the result of evaluating FullPostfixExpression
Let args be the result of evaluating Arguments
Return the result of callReference(ref,args)
FullPostfixExpression : PostfixExpression [no line break] ++
Let ref be the result of evaluating PostfixExpression
Let val be the result of readReference(ref)
Let num1 be the result of Number(val)
Let num2 be the result of evaluating the expression num1 + 1
Call writeReference(ref,num2)
Return num1
FullPostfixExpression : PostfixExpression [no line break] -
Let ref be the result of evaluating PostfixExpression
Let val be the result of readReference(ref)
Let num1 be the result of Number(val)
Let num2 be the result of evaluating the expression num1 - 1
Call writeReference(ref,num2)
Return num1
14.11 New expressions
A new expression results in the invocation of the intrinsic construct method of the value
computed by the expression that follows the new keyword. Arguments, if specified, are passed to the construct method. If no arguments are specified, the parentheses may be
omitted.
Syntax
FullNewExpression
new FullNewSubexpression Arguments
A FullNewExpression may be used as a FullPostfixExpression, or as
a FullNewSubexpression.
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77
FullNewSubexpression
PrimaryExpression
FullNewExpression
FullNewSubexpression PropertyOperator
SuperExpression PropertyOperator
A FullNewSubexpression may be used between the new keyword and the Arguments in a FullNewExpression, before a PropertyOperator in another FullNewSubexpression, or as
a ShortNewSubexpression.
ShortNewExpression
new ShortNewSubexpression
A ShortNewExpression may be used as a PostfixExpression, or as a
ShortNewSubexpression (that is, after the new keyword in another
ShortNewExpression.)
ShortNewSubexpression
FullNewSubexpression
ShortNewExpression
A ShortNewSubexpression may be used after the new keyword in
a ShortNewExpression.
Verification
FullNewExpression : new FullNewSubexpression Arguments
Let fun be the result of verifying FullNewSubexpression
Let args be the result of evaluating Arguments
If isStrict()
Call verifyType(fun,Function)
Let types be the value fun.types
If args.length is not equal to types.length, throw a VerifyError exception
For each type in args, call verifyType(args[i],types[i])
Return the result of calling resultType(fun,new)
FullNewSubexpression : PrimaryExpression
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78
FullNewSubexpression : FullNewExpression
Return the result of verifying the non-terminal symbol on the right-hand side of the production
Let ref be the result of evaluating ShiftExpression
Let x be the result of readReference(ref)
Let ref be the result of evaluating AdditiveExpression
Let y be the result of readReference(ref)
Return the result of shiftRightUnsigned(x,y)
14.16.4 Relational expressions
Syntax
RelationalExpressionallowIn
ShiftExpression
RelationalExpressionallowIn
< ShiftExpression
RelationalExpressionallowIn
> ShiftExpression
RelationalExpressionallowIn
<= ShiftExpression
RelationalExpressionallowIn
>= ShiftExpression
RelationalExpressionallowIn
in ShiftExpression
RelationalExpressionallowIn
instanceof ShiftExpression
RelationalExpressionallowIn
is ShiftExpression
RelationalExpressionallowIn
as ShiftExpression
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92
RelationalExpressionnoIn
ShiftExpression
RelationalExpressionnoIn
< ShiftExpression
RelationalExpressionnoIn
> ShiftExpression
RelationalExpressionnoIn
<= ShiftExpression
RelationalExpressionnoIn
>= ShiftExpression
RelationalExpressionnoIn
instanceof ShiftExpression
RelationalExpressionnoIn
is ShiftExpression
RelationalExpressionnoIn
as ShiftExpression
The noIn version of RelationalExpression exists to avoid ambiguity between the in operator in a relational expression and the inoperator in a for statement.
Let ref be the result of evaluating ListExpression
Return the result of readReference(ref)
15.3 Super statement
A SuperStatement causes the constructor of the immediate base class to be called. If
no SuperStatement is specified, the default constructor of the base class is called.
Unlike in Java, a SuperStatement may be used anywhere in the body of the constructor before an instance property is accessed. It is a compile error to use more than
one SuperStatement in a constructor.
Syntax
SuperStatement
super Arguments
A SuperStatement may only be used inside a constructor. It is a syntax error to use
a SuperStatement anywhere else in a program.
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110
class B extends A {
function B(x,y,z) {
super(x,y)
// other constructor code here
}
}
Semantics
Compatibility
In ActionScript 2.0, a SuperStatement may be used anywhere in a program, except in a
static method of a class. It is equivalent to the following statement:
If used in a class instance function, it will call the class's constructor function using the
current value of this as the first argument. If used in global code, it will call the global object's class's super constructor.
In ActionScript 3.0, a SuperStatement may only be used in an instance constructor. All
other uses will result in a syntax error. Also, if the number and types of Arguments is not compatible with Parameters of the super constructor, the result is a runtime error.
15.4 Block statement
Syntax
Block
{ Directives }
15.5 Labeled statement
Syntax
LabeledStatementω
Identifier : Substatementω
Verification
LabeledStatement : Substatement
Let breakTargets be the current set of possible targets of BreakStatements
Let target be the sequence of characters of Identifier
If target is a member of breakTargets, throw a SyntaxError
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111
Add target to breakTargets by calling breakTargets.push(target)
Verify Substatement
Evaluation
LabeledStatement : Substatement
Try
Return the result of evaluating Substatement
Catch exception x if x is of type Break
Let label be a string value consisting of the same sequence of characters as
Identifier
If x.target equals label, then return x.value
Throw x
Compatibility
ActionScript 2.0 does not allow LabeledStatements. This is a compatible change to the
language.
15.6 Conditional statements
15.6.1 If statement
Syntax
IfStatement abbrev
if ParenListExpression Substatement abbrev
if ParenListExpression Substatement noShortIf
else Substatement abbrev
IfStatement full
if ParenListExpression Substatement full
if ParenListExpression Substatement noShortIf
else Substatement full
IfStatement noShortIf
if ParenListExpression Substatement noShortIf
else Substatement noShortIf
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112
Verification
IfStatement : if ParenListExpression Substatement
IfStatement : if ParenListExpression Substatement else Substatement
Verify the non-terminal symbols other right-hand side of the production
Evaluation
IfStatement : if ParenListExpression Substatement
Let cv be a named argument passed to this evaluator
Let ref be the result of evaluating ParenListExpression
Let obj be the result of readReference(ref)
If toBoolean(obj) has the value true
Return the result of evaluating Substatement
Return cv
IfStatement : if ParenListExpression Substatement1 else Substatement2
Let cv be a named argument passed to this evaluator
Let ref be the result of evaluating ParenListExpression
Let obj be the result of readReference(ref)
If toBoolean(obj) has the value true
Return the result of evaluating Substatement1 passing the argument cv
Return the result of evaluating Substatement2 passing the argument cv
15.6.2 Switch statement
Syntax
SwitchStatement
switch ParenListExpression { CaseElements }
CaseElements
«empty»
CaseLabel
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113
CaseLabel CaseElementsPrefix CaseElement abbrev
CaseElementsPrefix
«empty»
CaseElementsPrefix CaseElement full
CaseElement ω
Directive ω
CaseLabel
CaseLabel
case ListExpression allowIn
:
default :
Semantics
Switch statements have the same syntax and semantics as defined in ECMA-262 edition
3.
15.7 Iteration statements
15.7.1 Do-while statement
Syntax
DoStatement
do Substatementabbrev while ParenListExpression
Verification
DoStatement : do Substatement while ParenListExpression
Let continueTargets be the current set of possible targets of continue targets
Let breakTargets be the current set of possible targets of break targets
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Add the label default to continueTargets by calling continueTargets.push(default)
Add the label default to breakTargets by calling breakTargets.push(default)
Verify Substatement
Verify ParenListExpression
Evaluation
DoStatement : do Substatement while ParenListExpression
Let cv be a named argument passed to this evaluator
Try
Loop
Try
Let cv be the result of evaluating Substatement with argument cv
Catch if exception x is of type Continue
If x.label is a member of the current loop's continueTargets, then cv = x.value
Throw x
Let ref be the result of evaluating ParenListExpression
Let obj be the result of readReference(ref)
If toBoolean(obj) is not true, then return cv
Catch if exception x is of type Break
If x.label equals default then return x.value
Throw x
15.7.2 While statement
Syntax
WhileStatementw
While ParenListExpression Substatementw
Verification
WhileStatement : while ParenLIstExpression Substatement
Let continueTargets be the current set of possible targets of continue targets
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Let breakTargets be the current set of possible targets of break targets
Add the label default to continueTargets by calling continueTargets.push(default)
Add the label default to breakTargets by calling breakTargets.push(default)
Verify ParenListExpression
Verify Substatement
Evaluation
DoStatement : do Substatement while ParenListExpression
Let cv be a named argument passed to this evaluator
Try
Loop
Let ref be the result of evaluating ParenListExpression
Let obj be the result of readReference(ref)
If toBoolean(obj) is not true, then return cv
Try
Let cv be the result of evaluating Substatement with argument cv
Catch if exception x is of type Continue
If x.label is a member of the current loop's continueTargets, then cv = x.value
Throw x
Catch if exception x is of type Break
If x.label equals default then return x.value
Throw x
15.7.3 For statements
Syntax
ForStatementw
For ( ForInitializer ; OptionalExpression ; OptionalExpression ) Substatementw
For ( ForInBinding in ListExpressionallowIn ) Substatementw
For [no line break] each ( ForInBinding in ListExpressionallowIn ) Substatementw
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ForInitializer
«empty»
ListExpressionnoIn
VariableDefinitionnoIn
ForInBinding
PostfixExpression
VariableDefinitionKind VariableBindingnoIn
OptionalExpression
ListExpressionallowIn
«empty»
Semantics
For statements in edition 4 have the same syntax and semantics as defined in ECMA-
262 edition 3 and E4X
15.8 Continue statement
Syntax
ContinueStatement
Continue
continue [no line break] Identifier
Verification
ContinueStatement : continue
Let continueTargets be the current set of possible continue targets
If default is not a member of continueTargets, throw a SyntaxError
ContinueStatement : continue Identifier
Let continueTargets be the current set of possible continue targets
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Let label be the sequence of characters of Identifier
If label is not a member of continueTargets, throw a SyntaxError
Evaluation
ContinueStatement : continue
Let cv be a named argument passed to this evaluator
Throw the exception Continue(cv,default)
ContinueStatement : continue Identifier
Let cv be a named argument passed to this evaluator
Let label be the sequence of characters of Identifier
Throw the exception Continue(cv,label)
Compatibility
ActionScript 2.0 does not allow the second form of ContinueStatement. This is a compatible change.
15.9 Break statement
Syntax
BreakStatement
break
break [no line break] Identifier
Verification
BreakStatement : break
Let breakTargets be the current set of possible break targets
If default is not a member of breakTargets, throw a SyntaxError
BreakStatement: break Identifier
Let breakTargets be the current set of possible continue targets
Let label be the sequence of characters of Identifier
If label is not a member of breakTargets, throw a SyntaxError
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Evaluation
BreakStatement: break
Let cv be a named argument passed to this evaluator
Throw the exception Break(cv,default)
BreakStatement: break Identifier
Let cv be a named argument passed to this evaluator
Let label be the sequence of characters of Identifier
Throw the exception Break(cv,label)
Compatibility
ActionScript 2.0 does not allow the second form of BreakStatement. This is a
compatible change.
15.10 With statement
Syntax
WithStatementw
with ParenListExpression Substatementw
Semantics
With statements have the same syntax and semantics as defined in ECMA-262 edition
3.
15.11 Return statement
Syntax
ReturnStatement
Return
return [no line break] ListExpressionallowIn
Verification
ReturnStatement : return
Let env be the lexical environment
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If env does not contain a parameter frame
Throw a SyntaxError exception
ReturnStatement : return ListExpression
Let env be the lexical environment
If env does not contain a parameter frame
Throw a SyntaxError exception
Let frame be the enclosing parameter frame
If frame does not allow a return value
Throw a SyntaxError exception
Verify ListExpression
Evaluation
BreakStatement: return
Throw the exception Return(undefined)
BreakStatement: return ListExpression
Let ref be the result of evaluating ListExpression
Let obj be the result of readReference(ref)
Throw the exception Return(obj)
15.12 Throw statement
Syntax
ThrowStatement
throw [no line break] ListExpressionallowIn
Verification
ThrowStatement : throw ListExpression
Verify ListExpression
Evaluation
ThrowStatement : throw ListExpression
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Let ref be the result of evaluating ListExpression
Let cv be a named argument passed to this evaluator
Try
Let cv be the result of evaluating Block1 with argument cv
Catch if exception x is of type Object (note: excludes Return, Break and Continue
exceptions)
Try
Let val be the result evaluating CatchClauses
If val is not none, then let e be have the value of x
Else let e be none
Catch if exception x
Let e have the value of x
Evaluate Block2
If e is not equal to none, then throw e, else return val
CatchClausesOpt : empty
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Return none
CatchClausesOpt : CatchClauses
Return the result of evaluating CatchClauses
CatchClauses : CatchClause
Return the result of evaluating CatchClause
CatchClauses : CatchClauses CatchClause
Let val be the result of evaluating CatchClauses
If val is not equal to none, then return val
Return the result of evaluating CatchClause
CatchClause : catch ( Parameter ) Block
Let env be a copy of the current lexical environment
Let x be the named argument of this evaluator
Let T be the type of Parameter
Let name be the name of Parameter
If x is of type T
Let scope be instance of the activation frame of CatchClause
Add scope to the lexical environment env
Call writeProperty(scope,name,x)
Return the result of evaluating Block
Return none
15.14 Default XML namespace statement
Syntax
DefaultXMLNamespaceStatement
default [no line break] xml [no line break] namespace = NonAssignmentExpressionb
Semantics
DefaultXMLNamespaceStatement sets the internal DefaultXMLNamespace property to the value of NonAssignmentExpression. If a DefaultXMLNamespaceStatement appears
in a function definition, the default xml namespace associated with the corresponding
function object is initially set to the unnamed namespace.
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16 Directives
Syntax
Directivew
EmptyStatement
Statementw
AnnotatableDirectivew
Attributes [no line break] AnnotatableDirectivew
IncludeDirective Semicolonw
ImportDirective Semicolonw
UseDirective Semicolonw
AnnotatableDirectivew
VariableDefinitionallowIn Semicolonw
FunctionDefinition
ClassDefinition
InterfaceDefinition
NamespaceDefinition Semicolonw
Directives
«empty»
DirectivesPrefix Directiveabbrev
DirectivesPrefix
«empty»
DirectivesPrefix Directivefull
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16.1 Attributes
Syntax
Attributes
Attribute
AttributeCombination
AttributeCombination
Attribute [no line break] Attributes
Attribute
AttributeExpression
ReservedNamespace
[ AssignmentExpressionallowIn ]
AttributeExpression
Identifier
AttributeExpression PropertyOperator
An AttributeExpression may be used as an Attribute.
An Attribute of one kind or another may be used before
any AnnotatableDirective. AnnotatableDirectives include variable, function, class, interface, and namespace definitions.
Here is a complete list of reserved attribute names:
public
private
internal
protected
override
final
dynamic
native
static
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Semantics
The meaning of an Attribute depends on its compile-time value and its usage. See the
description of the definitions being modified by the attribute.
16.2 Import directive
Syntax
ImportDirective
import PackageName . *
import PackageName . Identifier
ImportDirective may be used where ever a Directive or AnnotatableDirective can be
used.
import a.b.*
import a.b.x
Semantics
An ImportDirective causes the simple and fully qualified names of one or more public
definitions of the specified package to be introduced into the current package. Simple names will be shadowed by identical locally defined names. Ambiguous references to
imported names result in runtime errors.
The wildcard form (import a.b.*) imports all public names in a package. The single
name form (import a.b.x) imports only the specified name.
The mechanism for locating and loading imported packages is implementation defined.
Compatibility
The ActionScript 2.0 behavior of raising an error if there are two classes with the same simple name being imported is deprecated. ActionScript 3.0 will import both classes,
but references to the shared simple class name will result a compile-time error. Such
references must be disambiguated by using a fully qualified class name.
The ActionScript 2.0 behavior of implicit import is also deprecated and will result in a compile-time error in ActionScript 3.0. To work around such errors, an explicit import
directive must be added to the current package, which imports the referenced class.
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16.3 Include directive
Syntax
IncludeDirective
include [no line break] String
An IncludeDirective may be used where ever a Directive may be used.
include "reusable.as"
Semantics
An IncludeDirective results at compile time in the replacement of the text of the
IncludeDirective with the content of the stream specified by String.
Compatibility
In ActionScript 2.0, the include keyword is spelled #include. This form is deprecated
and results in a compile warning in ActionScript 3.0.
16.4 Use directive
Syntax
UseDirective
use namespace ListExpressionallowIn
A UseDirective may be used where ever a Directive or AnnotatableDirective may be
used. This includes the top-level of a Program,PackageDefinition and ClassDefinition.
use namespace ns1, ns2
Semantics
A UseDirective causes the specified namespaces to be added to the open namespaces and removed when the current block scope is exited. Each sub expression of
ListExpression must have a compile-time constant Namespace value.
Compatibility
UseDirective is an extension to ActionScript 2.0.
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17 Definitions
17.1 Variable definition
Syntax
VariableDefinitionb
VariableDefinitionKind VariableBindingListb
VariableDefinitionKind
var
const
VariableBindingListb
VariableBindingb
VariableBindingListb , VariableBindingb
VariableBindingb
TypedIdentifierb VariableInitialisationb
VariableInitialisationb
«empty»
= VariableInitialiserb
VariableInitialiserb
AssignmentExpressionb
AttributeCombination
TypedIdentifierb
Identifier
Identifier : TypeExpressionb
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TypedIdentifier may be used in a VariableBinding or Parameter definition.
var x : String = "initial String value of var x"
function plusOne( n : Number ) { return n + 1 }
Semantics
TypedIdentifer results at compile-time in a variable or parameter that is optionally
typed. The TypeExpression, if given, results at compile-time to a Type value. It is used
to specify the set of values that are compatible with the variable or parameter being declared.
A VariableDefinition may be modified by the following attributes
static adds property to the class object
prototype adds property to the prototype object
private accessible from within the current class
public accessible outside the current package
protected accessible from within an instance of the current class or a derived classes
internal accessible from within the current package
Compatibility
Typed identifier behavior differs between ActionScript 3.0 and ActionScript 2.0 in two
ways. ActionScript 2.0 checks for type compatibility using compile-time types at compile time, while ActionScript 3.0 checks for type compatibility using runtime types
at runtime. The difference can be seen in the following examples:
var s : String = o
function f( s : String ) {}
var o = 10
f(o) // OK in ActionScript 2.0, error in ActionScript 3.0
In ActionScript 2.0, the variable o does not have an explicit compile-time type that can be compared to the type String of the parameter s in the call to function f, so no error
is reported. In ActionScript 3.0, the value of argument o is compared to the type of the
parameter s at runtime, resulting in an error.
class A {}
class B extends A { var x }
var a : A = new B
a.x = 20 // Error in ActionScript 2.0, OK in ActionScript 3.0 (since
instance of B has an x property)
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In ActionScript 2.0, the compiler uses A, the declared type of a, to conservatively check
for valid uses of a, excluding completely safe and reasonable uses of a. In ActionScript 3.0, the compiler uses the type of a to optimize its use, but does not report type errors.
It leaves that task to the runtime.
17.2 Function definition
Syntax
FunctionDefinition
function FunctionName FunctionCommon
Semantics
A FunctionDefinition introduces a new name and binds that name to a newly created
function object specified by FunctionCommon. The implementation of the function
object depends on whether the function is static or virtual as indicated by its context and attributes.
A FunctionDefinition may be modified by the following attributes
static Adds property to the class object
prototype Adds property to the prototype object
final Adds non-overridable property to each instance
override Overrides a method of the base class
private Accessible from within the current class
public Accessible outside the current package
protected Accessible from within an instance of the current or a derived classes
internal Accessible from within the current package
native Generates a native stub (implementation defined)
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Syntax
FunctionName
Identifier
get [no line break] Identifier
set [no line break] Identifier
FunctionName is used inside a FunctionDefinition.
function f() {}
function get x () { return impl.x }
function set x (x) { impl.x = x }
Semantics
FunctionName specifies at compile time the name and kind of function being defined. A
name that includes a get or set modifier specifies that the function being defined is a property accessor.
17.2.1 Function body
Syntax
FunctionCommon
FunctionSignature
FunctionSignature Block
Verification
A FunctionCommon that is a FunctionSignature without a Block introduces an abstract method trait. A FunctionCommon with a FunctionSignature followed by a Block defines
a concrete function. The result of verifying a FunctionCommon node is the addition of a
method trait to a set of traits associated with an object at runtime.
Evaluation
During evaluation, a FunctionCommon node is instantiated and activated. Function
instantiation is when a lexical environment is associated with a function object. This
captured environment is used to activate the function. Activation is when the function is called with a specific receiver (this) and set of arguments.
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17.2.2 Function signature
Syntax
FunctionSignature
( ) ResultType
( Parameters ) ResultType
Semantics
The function signature defines the set of traits associated with the activation of a
function object.
17.2.3 Parameter list
In the strict dialect, the Arguments assigned to Parameters must have compatible
number and types. In the standard dialect, the handling of arguments is the same as
If val is equal to none, then throw a VerifyError: must be a compile-time constant
Call defineSlotTrait(frame,name,type,val,false)
RestParameter : …
Do nothing
RestParameter : … Identifier
Let frame be a named argument passed into this verifier
Verify Identifier
Let name be the name of Identifier
Call defineSlotTrait(frame,name,Array,false)
17.2.4 Result type
Syntax
ResultType
«empty»
: TypeExpressionallowIn
ResultType may be used in a FunctionSignature.
function f(x) : Number { return x }
Semantics
ResultType guarantees the type of the value returned from a function. It is a verify
error if the return value does not implicitly convert to the ResultType of the function.
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Compatibility
The ActionScript 2.0 behavior of checking types only at compile time is more permissive
than in ActionScript 3.0. This will result in new runtime errors in cases such as calling the method shown above with an argument of type String.
17.3 Class definition
Syntax
ClassDefinition
Class ClassName Inheritance Block
ClassDefinition may be used where ever an AnnotatableDirective may be used, which
includes wherever a Directive can be used and following a list of attributes, except inside of another ClassDefinition or InterfaceDefinition.
class A extends B implements C {}
dynamic public final class D {}
17.3.1 Class attributes
Class definitions may be modified by the following attributes:
internal Visible to references inside the current package (default)
public Visible to references everywhere
final Prohibit extension by sub-classing
dynamic Allow the addition of dynamic properties
The default attributes for a class definition are internal, non-dynamic, and non-final.
Semantics
A class definition adds a new class name into the current scope. In the following
drawing, the class name A refers to a class object with the structure shown in the
drawing:
class A {}
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A class definition causes a class object and prototype instance to be created. The
default delegate of the instance prototype is the Object prototype. The default super
class of the class object is the Object class. Static members are added to the class object as fixed properties, and non-static members are added to the instance prototype
as fixed properties. The internal references (traits, prototype, constructor, and
delegate) between these objects are read-only.
17.3.2 Class name
Syntax
ClassName
ClassIdentifiers
ClassIdentifiers
Identifier
ClassIdentifiers . Identifier
ClassName can be used in ClassDefinition.
class A {}
Semantics
ClassName evaluates at compile time to a type name.
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Compatibility
The form ClassIdentifiers : ClassIdentifiers . Identifier is deprecated. It is equivalent to
declaring the class name Identifier in the package ClassIdentifiers.
A ClassDefinition may extend another class definition and implement one or more
interfaces. We say that a class inherits the properties of its base class and the abstract
methods of its interfaces. When a class extends another class, it is inherits the base class's instance properties, but not its static properties. When a class implements one
or more interfaces it is required to define each inherited interface method.
The TypeExpressions that occur in the extends and implements clauses must be
compile-time constant expressions without forward references.
17.3.4 Class block
Syntax
The body of a class definition is syntactically a Block. The class block must come
immediately after the ClassName or Inheritance constituents, if present. The class block must not contain a ClassDefinition or InterfaceDefinition.
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Semantics
Declarations modified by the static attribute contribute properties to the class object;
declarations without the static attribute contribute properties to the instance traits object. Statements that are not declarations are evaluated normally when the class
object is instantiated.
17.3.4.1 Variables
The following attributes are allowed in variable definitions in a class block:
static Defines a property of the class object
private Visible to references inside the current class
internal Visible to references inside the current package
protected Visible to references inside an instance of the current class and derived classes
prototype Defines a property of the class prototype object
public Visible to references everywhere
AttributeExpression Namespace value is the qualifier for the name of the definition
The default attributes for variable definitions are non-static and internal.
17.3.4.2 Methods
The following attributes are allowed in function definitions in a class block:
static Defines a property of the class object
final Must not be overridden
override Must redefine an inherited non-final method
native Implementation defined
private Visible to references inside the current class
internal Visible to references inside the current package
protected Visible to references inside instances of the current class and derived classes
public Visible to references everywhere
AttributeExpression Namespace value is the qualifier for the name of the definition
The default attributes for function definitions in a class are non-static , non-final, non-
native and internal.
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Methods that implement interface methods must be instance methods defined with
attributes that include public. Interface methods may be overridden in a derived class as long as the overriding method also has the public attribute.
A constructor method is a method with the same name as the class it is defined in. It is
a syntax error for the constructor method to have a different namespace attribute than its class.
It is a verifier error for override to appear as an attribute of a class method that does
not override another method.
17.4 Interface definition
Syntax
InterfaceDefinition
interface ClassName ExtendsList Block
An InterfaceDefinition may be used where ever a Directive or AnnotatableDirective may be used, which includes wherever a Directive can be used and following a list of
attributes, except inside of another ClassDefinition or InterfaceDefinition.
interface T { function m() }
Semantics
An InterfaceDefinition constrains the structure of any ClassDefinition that implements
it. These constraints are enforced when the ClassDefinition is being compiled.
An InterfaceDefinition also introduces a new type name into the current scope. When evaluated in a context that expects a type value, a reference to that name is equivalent
to the set of types comprising the types of all instances of all classes that implement
the interface.
Compatibility
In ActionScript 2.0, user-defined types only exist at compile time. Therefore, any use of
an interface name that cannot be enforced at compile time will have no effect on the
program. See descriptions of ResultType and TypeIdentifier.
17.4.1 Interface attributes
Interface definitions may be modified by these attributes
internal Visible to references inside the current package (default)
public Visible to references everywhere
The default modifier for an interface definition is internal.
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17.4.2 Interface name
The name of an interface definition has the syntax and semantics of a ClassName
(section 16.3.1).
17.4.3 Interface inheritance
Syntax
ExtendsList
«empty»
extends TypeExpressionList
An ExtendsList may be used after the ClassName and before Block in an InterfaceDefinition.
interface U extends T { function n() }
Semantics
An ExtendsList specifies the interfaces that include instances of the current InterfaceDefinition in their value set. It also specifies that the current
InterfaceDefinition inherits the structure of each of the interfaces named in the
ExtendsList.
An interface definition must not introduce a method with a name that has the same
identifier as an inherited method.
An interface definition must not inherit itself directly or indirectly.
17.4.4 Interface block
The body of an interface definition is syntactically a Block, but must only contain
FunctionDefinitions with no Block and no attribute.
17.4.4.1 Interface methods
Interface methods must be defined with no attribute. An interface method is given the
name that has its interface as its qualifier and the identifier as the string.
Interface methods have the syntax of a FunctionDefinition without the Block of
FunctionCommon. Class methods that implement interface methods must match the name and signature, including parameter count, types and result type, exactly. The
name of the implementing method must have a name that is qualified by the public
namespace.
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17.5 Package definition
Syntax
PackageDefinition
package PackageNameOpt Block
A PackageDefinition may be used in a Program before any Directive that is not
a PackageDefinition is used.
package p {
public class A {}
public interface I {}
}
package q {
public var x = 10
}
import p.*
import q.f
import y = q.x
class B extends A implements I {}
q.f()
trace(x)
Semantics
A PackageDefinition introduces a new package name into the current scope. A package definition causes the public members of that package to be qualified by the package
name, and the internal members of that package definition to be qualified by an
anonymous namespace that is only accessible to code inside the package.
The statements of a package body are executed in the global scope of the Program.
Compatibility
PackageDefinition is an extension to ActionScript 2.0. It is added to ActionScript 3.0 to
replace the deprecated form of ClassDefinition that uses a ClassName qualified by a package name.
17.5.1 Package name
Syntax
PackageName
Identifier
PackageName . Identifier
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17.6 Namespace definition
Syntax
NamespaceDefinition
namespace NamespaceBinding
NamespaceBinding
Identifier NamespaceInitialisation
NamespaceInitialisation
«empty»
= AssignmentExpressionallowIn
A NamespaceDefinition may be used where ever a Directive or AnnotatableDirective may be used. This includes the top-level of
A NamespaceDefinition introduces a new namespace constant into the current block
scope and assigns to it either an anonymous namespace value, or the value of the
AssignmentExpression in the NamespaceInitialisation implicitly coerced to type Namespace. The value of NamespaceInitialisation must be a compile-time constant with
a value of type String or type Namespace.
NamespaceDefinitions can be annotated by an access specifier (private, internal, protected or public), the static modifier inside
aClassDefinition.
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17.7 Program definition
Syntax
Program
Directives
PackageDefinition Program
package P {
function f() {}
}
package Q {
function f() {}
}
P.f()
Q.f()
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18 Errors
18.1 Class errors
The following errors may occur while parsing or verifying a class definition:
Defining a class with the name of another definition in the same scope
Defining a class that extends itself directly or indirectly
Defining a constructor with a namespace attribute that is different than the
namespace attribute of its class
Defining a constructor with a result type
Defining a constructor that calls its super constructor more than once
Defining a constructor that calls its super constructor accessing a non-local property
Introducing a method or variable with the same name as an inherited method or
variable
Overriding a variable
Overriding a final method
Overriding a method that is not defined in a base class
Overriding a method with a method that has a different number, types of
parameters, or result type
18.2 Interface errors
The following errors may occur while parsing or verifying an interface definition:
Defining an interface with the name of another definition in the same scope
Defining an interface that extends itself directly or indirectly
Defining an interface with a body that contain a definition or statement other than
a function definition with no block
Defining an interface method with the same identifier as an inherited interface
method
Defining an interface method with an attribute
18.3 Package errors
The following list describes package errors:
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It is a parser error to define a package inside a package.
It is a parser error to use attributes on a package definition.
It is a parser error to import a package's names into itself
It is a strict error to import the same name more than once into the same package.
It is a strict error to import a package that cannot be found.
It is a strict error to reference a package property that cannot be found in an
imported package.
18.4 Namespace errors
The following list describes namespace errors:
It is a verifier error to use an expression that does not have a compile-time constant namespace value in a use namespace directive.
It is a verifier error to use an attribute expression that is not a compile-time
constant namespace value as a definition attribute.
It is a verifier error to use a user-defined namespace as an attribute except to
define a class or instance property.
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19 Native objects
The form and function of the native objects is the same as ECMA-262 edition 3 except that all prototype properties are also implemented as class methods. Prototype
properties that are functions are implemented as regular methods. Prototype properties
that are variables are implemented as a pair of get and set methods that forward state to the prototype property.
19.1 Global object
Global object
NaN
Infinity
undefined
parseInt
parseFloat
isNaN
isFinite
decodeURI
decodeURIComponent
encodeURI
encodeURIComponent
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19.2 Object objects
Object object
Object
Object.prototype
Object.prototype.constructor
Object.prototype.toString
Object.prototype.toLocaleString
Object.prototype.valueOf
Object.prototype.hasOwnProperty
Object.prototype.isPrototypeOf
Object.prototype.propertyIsEnumerable
19.3 Function objects
Function object
Function
Function.prototype
Function.prototype.constructor
Function.prototype.toString
Function.prototype.apply
Function.prototype.call
Function.length
Function.prototype
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19.4 Array objects
Array object
Array
Array.prototype
Array.prototype.constructor
Array.prototype.toString
Array.prototype.toLocaleString
Array.prototype.concat
Array.prototype.join
Array.prototype.pop
Array.prototype.push
Array.prototype.reverse
Array.prototype.shift
Array.prototype.slice
Array.prototype.sort
Array.prototype.splice
Array.prototype.unshift
Array.[[Put]]
Array.length
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19.5 String objects
String object
String
String.prototype
String.fromCharCode
String.prototype.constructor
String.prototype.toString
String.prototype.valueOf
String.prototype.charAt
String.prototype.charCodeAt
String.prototype.concat
String.prototype.indexOf
String.prototype.lastIndexOf
String.prototype.localeCompare
String.prototype.match
String.prototype.replace
String.prototype.search
String.prototype.slice
String.prototype.split
String.prototype.substring
String.prototype.toLowerCase
String.prototype.toLocaleLowerCase
String.protoype.toUpperCase
String.protoype.toLocaleUpperCase
String.[[Value]]
String.length
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19.6 Boolean objects
Boolean object
Boolean
Boolean.prototype
Boolean.prototype.constructor
Boolean.prototype.toString
Boolean.prototype.valueOf
19.7 Number objects
Number object
Number
Number.prototype
Number.MAX_VALUE
Number.MIN_VALUE
Number.NaN
Number.NEGATIVE_INFINITY
Number.POSITIVE_INFINITY
Number.protoype.constructor
Number.protoype.toString
Number.prototype.toLocaleString
Number.prototype.valueOf
Number.prototype.toFixed
Number.prototype.toExponential
Number.prototype.toPrecision
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19.8 Math object
Math object
Math.E
Math.LN10
Math.LN2
Math.LOG2E
Math.LOG10E
Math.PI
Math.SQRT1_2
Math.SQRT2
Math.abs
Math.acos
Math.asin
Math.atan
Math.atan2
Math.ceil
Math.cos
Math.exp
Math.floor
Math.log
Math.max
Math.min
Math.pow
Math.random
Math.round
Math.sin
Math.sqrt
Math.tan
19 Native objects
151
19.9 Date objects
Date object
Date
Date.protoype
Date.parse
Date.UTC
Date.prototype.constructor
Date.prototype.toString
Date.prototype.toDateString
Date.prototype.toTimeString
Date.prototype.toLocaleString
Date.prototype.toLocaleDateString
Date.prototype.toLocaletimeString
Date.prototype.valueOf
Date.prototype.getTime
Date.prototype.getFullYear
Date.prototype.getUTCFullYear
Date.prototype.getMonth
Date.prototype.getUTCMonth
Date.prototype.getDate
Date.prototype.getUTCDate
Date.prototype.getDay
Date.prototype.getUTCDay
Date.prototype.getHours
Date.prototype.getUTCHours
Date.prototype.getMinutes
Date.prototype.getUTCMinutes
Date.prototype.getSeconds
19 Native objects
152
Date object
Date.prototype.getUTCSeconds
Date.prototype.getMilliseconds
Date.prototype.getUTCMilliseconds
Date.prototype.getTimezoneOffset
Date.prototype.setTime
Date.prototype.setMilliseconds
Date.prototype.setUTCMilliseconds
Date.prototype.setSeconds
Date.prototype.setUTCSeconds
Date.prototype.setMinutes
Date.prototype.setUTCMinutes
Date.prototype.setHours
Date.prototype.setUTCHours
Date.prototype.setDate
Date.prototype.setUTCDate
Date.prototype.setMonth
Date.prototype.setUTCMonth
Date.prototype.setFullYear
Date.prototype.setUTCFullYear
Date.prototype.toUTCString
19 Native objects
153
19.10 Error objects
Error object
Error
Error.prototype
Error.prototype.constructor
Error.prototype.name
Error.prototype.message
Error.prototype.toString
20 Compatibility with the static profile
154
20 Compatibility with the static profile
The static profile defines a dialect that is a subset of ActionScript 3.0. It allows for the static interpretation of type names and the reporting of verifier errors ahead-of-time.
21 Compatibility with ECMAScript edition 3
155
21 Compatibility with ECMAScript edition 3
While we have made this edition as compatible as possible with the ECMAScript (ECMA-262) edition 3 language specification, there are certain behaviors for which there is no
clear use case and keeping them as-is would have been placed an unneeded heavy
burden on the new features of the language. In such cases, we have made small and calculated changes to allow the new definition to be simpler and easier to use.
21.1 this inside of nested function
In ECMA-262 edition 3, when this appears in a nested function, it is bound to the
global object if the function is called lexically, without an explicit receiver object. In ActionScript 3.0, this is bound to the innermost nested this when the function is
called lexically.
21.2 No boxing of primitives
In ECMA-262 edition 3, primitive values (Boolean, Number, String) are boxed in Object
values in various contexts. In ActionScript 3.0, primitives are permanently sealed
Objects. Unlike boxed objects, attempts to dynamically extend a sealed object results in a runtime exception.
21.3 Assignment to const is a runtime exception
In ECMA-262 edition 3, primitive assignment to read-only properties failed silently. In
ActionScript 3.0, primitive assignment to read-only properties causes a runtime error to be thrown.
21.4 Class names are const
In ECMA-262 edition 3, constructor functions were writable. In ActionScript 3.0, we implement these properties with class definitions, which are read only.
21.5 Array arguments object
In ECMA-262 edition 3, the function arguments property is a generic Object. In
ActionScript 3.0, arguments is an Array.
22 Compatibility with E4X
156
22 Compatibility with E4X
We have made ActionScript 3.0 as compatible as possible with the ECMAScript for XML (E4X) specification (ECMA-357 edition 2). ActionScript 3.0 diverges from the E4X
specification in a small number of cases due to minor errors in the current edition of the
